Fig. 1. Section through the anterior part of an embryo ofScyllium caniculaduring stage L.
c.Peculiar large cells which are found at the dorsal part of the spinal cord. Sympathetic ganglion shewn atsyg. Zeiss A,ocul.1.
Fig. 2. Section through the lateral line at the time of its first formation.
The cells markednlwere not sufficiently distinct to make it quite certain that they really formed part of the lateral nerve. Zeiss B,ocul.2.
Figs. 3a, 3b, 3c, 3d. Four sections of the lateral line from an embryo belonging to stage L. 3ais the most anterior. In 3athe lateral nerve (nl) is seen to lie in the mesoblast at some little distance from the lateral line. In 3band 3cit lies in immediate contact with and partly enclosed by the modified epiblast cells of the lateral line. In 3d, the hindermost section, the lateral line is much larger than in the other sections, but no trace is present of the lateral nerve. The sections were taken from the following slides of my series of the embryo (the series commencing at the tail end) 3d(46), 3c(64), 3b(84), 3a(93). The figures all drawn on the same scale, but 3ais not from the same side of the body as the other sections.
Fig. 4. Section through lateral line of an embryo of stage P at the point where it is acquiring an opening to the exterior. The peculiar modified cells of its innermost part deserve to be noticed. Zeiss D,ocul.2.
Fig. 5. Mucous canals of the head with branches of the ramus ophthalmicus growing towards them. Stage O. Zeiss A,ocul.2.
Fig. 6. Mucous canals of head with branches of the ramus ophthalmicus growing towards them. Stage between O and P. Zeiss a a,ocul.2.
Fig. 7. Junction of a nerve and mucous canal. Stage P. Zeiss D,ocul.2.
Fig. 8. Longitudinal and horizontal section through the muscle-plates and adjoining structures at a stage intermediate between L and M. The section is intended to shew the gradual conversion of the cells of the somatic layer of muscle-plates into muscles.
Fig. 9. Longitudinal section through the notochord and adjoining parts to shew the first appearance of the cartilaginous notochordal sheath which forms the vertebral centra. Stage N.
Fig. 10. Transverse section through the tail of an embryo of stage P to shew the coexistence of the rib-process and hæmal arches in the first few sections behind the point where the latter appear. Zeiss C,ocul.1.
Fig. 11. Transverse section through the centre of a caudal vertebra of an embryo somewhat older than Q. It shews (1) the similarity between the arch-tissue and the hyaline tissue of the outer layer of the vertebral centrum, and (2) the separation of the two by the membrana elastica externa[2](mel). It shews also the differentiation of three layers in the vertebral centrum:videp.374.
[2]The slight difference observable between these two tissues in the arrangement of their nuclei has been much exaggerated by the engraver.
EXPLANATION OFPLATE 14. (X.p.378.)
This Plate illustrates the Formation of the Spinal Nerves.
Complete List of Reference Letters.
ar.Anterior root of a spinal nerve.ch.Notochord.com.Commissure connecting the posterior roots of the spinal nerves.i.Mesoblastic investment of spinal cord.mp.Muscle-plate.n.Spinal nerve.nc.Neural canal.pr.Posterior root of a spinal nerve.spg.Ganglion on posterior root of spinal nerve.v.r.Vertebral rudiment.w.White matter of spinal cord.y.Point where the spinal cord became segmented off from the superjacent epiblast.
Figs. 1, 2, and 3. Three sections of a Pristiurus embryo belonging to stage I. Fig. 1 passes through the heart, fig. 2 through the anterior part of the dorsal region, fig. 3 through a point slightly behind this. (ZeissCC,ocul.2.) In fig. 3 there is visible a slight proliferation of cells from the dorsal summit of the neural canal. In fig. 2 this proliferation definitely constitutes two club-shaped masses of cells (pr)—the rudiments of the posterior nerve-roots,—both attached to the dorsal summit of the spinal cord. In fig. 1 the rudiments of the posterior roots are of considerable length.
Fig. 4. Section through the dorsal region of a Torpedo embryo slightly older than stage I, with three visceral clefts. (ZeissCC,ocul.2.) The section shews the formation of a pair of dorsal nerve-rudiments (pr) and a ventral nerve-rudiment (ar). The latter is shewn in its youngest condition, and is not distinctly cellular.
Fig. 5. Section through the dorsal region of a Torpedo embryo slightly younger than stage K. (ZeissCC,ocul.2.) The connective-tissue cells are omitted. The rudiment of the ganglion (spg) on the posterior root has appeared, and the junction of posterior root with the cord is difficult to detect. The anterior root forms an elongated cellular structure.
Fig. 6. Section through the dorsal region of a Pristiurus embryo of stage K. (ZeissCC,ocul.2.) The section especially illustrates the attachment of the posterior root to the spinal cord.
Fig. 7. Section through the same embryo as fig. 6. (ZeissCC,ocul.1.) The section contains an anterior root, which takes its origin at a point opposite the interval between two posterior roots.
Fig. 8. A series of posterior roots with their central ends united by a dorsal commissure, from a longitudinal and vertical section of a Scyllium embryo belonging to a stage intermediate between L and M. The embryo was hardened in a mixture of osmic and chromic acids.
Fig. 9. The central end of a posterior nerve-root from the same embryo, with the commissure springing out from it on either side.
EXPLANATION OFPLATES 15,16,17. (X.p.397.)
Plate 15. (The Head during stages G—K.)
Complete List of Reference Letters.
1aa, 2aa, etc.1st,2d, etc. aortic arch.acv.Anterior cardinal vein.al.Alimentary canal.ao.Aorta.au.Thickening of epiblast to form the auditory pit.aun.Auditory nerve.aup.Auditory pit.auv.Auditory vesicle.b.Wall of brain.bb.Base of brain.cb.Cerebellum.cer.Cerebrum.Ch.Choroid slit.ch.Notochord.com.Commissure connecting roots of vagus nerve. 1, 2, 3 etc.eg.External gills.ep.External epiblast.fb.Fore-brain.gl.Glossopharyngeal nerve.hb.Hind-brain.ht.Heart.hy.Hyaloid membrane.In.Infundibulum.l.Lens.M.Mouth involution.m.Mesoblast at the base of the brain.mb.Mid-brain.mn.v.Mandibular branch of fifth.ol.Olfactory pit.op.Eye.opn.Optic nerve.opv.Optic vesicle.opthV. Ophthalmic branch of fifth.p.Posterior root of spinal nerve.pn.Pineal gland. 1, 2 etc.pp.First, second, etc. section of body-cavity in the head.pt.Pituitary body.so.Somatopleure.sp.Splanchnopleure.spc.Spinal cord.Th.Thyroid body.v.Blood-vessel.iv.v.Fourth ventricle. v. Fifth nerve.Vc.Visceral cleft.Vg.Vagus.vii.Seventh or facial nerve.
Fig. 1. Head of a Pristiurus embryo of stage K viewed as a transparent object.
The points which deserve special attention are: (1) The sections of the body-cavity in the head (pp): the first or premandibular section being situated close to the eye, the second in the mandibular arch. Above this one the fifth nerve bifurcates. The third at the summit of the hyoid arch.
The cranial nerves and the general appearance of the brain are well shewn in the figure.
The notochord cannot be traced in the living embryo so far forward as it is represented. It has been inserted according to the position which it is seen to occupy in sections.
Fig. 2. Head of an embryo of Scyllium canicula somewhat later than stage K, viewed as a transparent object.
The figure shews the condition of the brain; the branches of the fifth and seventh nerves (v.vii.); the rudiments of the semicircular canals; and the commencing appearance of the external gills as buds on both walls of 2nd, 3rd, and 4th clefts. The external gills have not appeared on the first cleft or spiracle.
Fig. 3. Section through the head of a Pristiurus embryo during stage G. It shews (1) the fifth nerve (v.) arising as an outgrowth from the dorsal summit of the brain. (2) The optic vesicles not yet constricted off from the fore-brain.
Figs. 4aand 4b. Two sections through the head of a Pristiurus embryo of stage I. They shew (1) the appearance of the seventh nerve. (2) The portion of the body-cavity belonging to the first and second visceral arches. (3) The commencing thickening of epiblast to form the auditory involution.
In 4b, the posterior of the two sections, no trace of an auditory nerve is to be seen.
Figs. 5aand 5b. Two sections through the head of a Torpedo embryo with 3 visceral clefts. Zeiss A,ocul.1.
5ashews the formation of the thin roof of the fourth ventricle by a divarication of the two lateral halves of the brain.
Both sections shew the commencing formation of the thyroid body (th) at the base of the mandibular arch.
They also illustrate the formation of the visceral clefts by an outgrowth from the alimentary tract without any corresponding ingrowth of the external epiblast.
Fig. 6. Section through the hind-brain of a somewhat older Torpedo embryo. Zeiss A,ocul.1.
The section shews (1) the attachment of a branch of the vagus to the walls of the hind-brain. (2) The peculiar form of the hind-brain.
Fig. 7. Transverse section through the head of a Pristiurus embryo belonging to a stage intermediate between I and K, passing through both the fore-brain and the hind-brain. Zeiss A,ocul.1.
The section illustrates (1) the formation of the pituitary body (pt) from the mouth involution (m), and proves that, although the wall of the throat (al) is in contact with the mouth involution, there is by this stage no communication between the two. (2) The eye. (3) The sections of the body-cavity in the head (1pp, 2pp). (4) The fifth nerve (v.) and the seventh nerve (vii).
Fig. 8. Transverse section through the brain of a rather older embryo than fig. 7. It shews the ventral junction of the anterior sections of the body-cavity in the head (1pp).
Figs. 9aand 9b. Two longitudinal sections through the brain of a Pristiurus embryo belonging to a stage intermediate between I and K. Zeiss A,ocul.1.
9ais taken through the median line, but is reconstructed from two sections. It shews (1) The divisions of the brain—The cerebrum and thalamencephalon in the fore-brain; the mid-brain; the commencing cerebellum in the hind-brain. (2) The relation of the mouth involution to the infundibulum. (3) The termination of the notochord.
9bis a section to one side of the same brain. It shews (1) The divisions of the brain. (2) The point of outgrowth of the optic nerves (opn). (3) The sections of the body-cavity in the head and the bifurcation of the optic nerve over the second of these.
Fig. 10. Longitudinal section through the head of a Pristiurus embryo somewhat younger than fig. 9. Zeiss a,ocul.4. It shews the relation of the nerves and the junction of the fifth, seventh, and auditory nerves with the brain.
Fig. 11. Longitudinal section through the fore-brain of a Pristiurus embryo of stage K, slightly to one side of the middle line. It shews the deep constriction separating the thalamencephalon from the cerebral hemispheres.
Fig. 12. Longitudinal section through the base of the brain of an embryo of a stage intermediate between I and K.
It shews (1) the condition of the end of the notochord; (2) the relation of the mouth involution to the infundibulum.
Fig. 13a. Longitudinal and horizontal section through part of the head of a Pristiurus embryo rather older than K. Zeiss A,ocul.1.
The figure contains the eye cut through in the plane of the choroid slit. Thus the optic nerve (opn) and choroid slit (ch) are both exhibited. Through the latter is seen passing mesoblast accompanied by a blood-vessel (v).Oprepresents part of the optic vesicle to one side of the choroid slit.
No mesoblast can be seen passing round the outside of the optic cup; and the only mesoblast which enters the optic cup passes through the choroid slit.
Fig. 13b. Transverse section through the last arch but one of the same embryo as 13a. Zeiss A,ocul.1.
The figure shews (1) The mode of formation of a visceral cleft without any involution of the external skin. (2) The head-cavity in the arch and its situation in relation to the aortic arch.
Fig. 14. Surface view of the nasal pit of an embryo of same age as fig. 13, considerably magnified. The specimen was prepared by removing the nasal pit, flattening it out and mounting in glycerine after treatment with chromic acid. It shews the primitive arrangement of the Schneiderian folds. One side has been injured.
Figs. 15aand 15b. Two longitudinal and vertical sections through the head of a Pristiurus embryo belonging to stage K. Zeiss a,ocul.3.
15ais the most superficial section of the two. It shews the constitution of the seventh and fifth nerves, and of the intestinal branch of the vagus. The anterior branch of the seventh nerve deserves a special notice.
15bmainly illustrates the dorsal commissure of the vagus nerve (com) continuous with the dorsal commissures of the posterior root of the spinal nerves.
Fig. 16. Two longitudinal and vertical sections of the head of a Pristiurus embryo belonging to the end of stage K. Zeiss a,ocul.1.
16apasses through the median line of the brain and shews the infundibulum, notochord and pituitary body, etc.
The pituitary body still opens into the mouth, though the septum between the mouth and the throat is broken through.
16bis a more superficial section shewing the head-cavitiespp1, 2, 3, and the lower vagus commissure.
Plate 16.(X.p.397.)
Complete List of Reference Letters.
auv.Auditory vesicle.cb.Cerebellum.cer.Cerebral hemispheres.ch.Notochord.cin.Internal carotid.ft.Fasciculi teretes.in.Infundibulum.lv.Lateral ventricle.mb.Mid-brain, or optic lobes.md.Medulla oblongata.mn.Mandible.ol.Olfactory pit.oll.Olfactory lobe.op.Eye.opn.Optic nerve.opth.Optic thalamus.pc.Posterior commissure.pcl.Posterior clinoid.pn.Pineal gland.pt.Pituitary body.rt.Restiform tracts.tv.Tela vasculosa of the roof of the fourth ventricle.iv.v.Fourth ventricle.vii.Seventh nerve.x.Rudiment of septum which will grow backwards and divide the unpaired cerebral rudiment into the two hemispheres.
Figs. 1a, 1b, 1c. Longitudinal sections of the brain of a Scyllium embryo belonging to stage L. Zeiss a,ocul.1.
1ais taken slightly to one side of the middle line, and shews the general features of the brain, and more especially the infundibulum (in) and pituitary body (pt).
1bis through the median line of the pineal gland.
1cis through the median line of the base of the brain, and shews the notochord (ch) and pituitary body (pt); the latter still communicating with the mouth. It also shews the wide opening of the infundibulum in the middle line into the base of the brain.
Fig. 2. Section through the unpaired cerebral rudiment during stage O, to shew the origin of the olfactory lobe and the olfactory nerve. The latter is seen to divide into numerous branches, one of which passes into each Schneiderian fold. At its origin are numerous ganglion cells represented by dots. Zeiss a,ocul.2.
Fig. 3. Horizontal section through the three lobes of the brain during stage O. Zeiss a,ocul.2.
The figure shews (1) the very slight indications which have appeared by this stage of an ingrowth to divide the cerebral rudiment into two lobes (x): (2) the optic thalami united by a posterior commissure, and on one side joining the base of the mid-brain, and behind them the pineal gland: (3) the thin posterior wall of the cerebral rudiment with folds projecting into the cerebral cavity.
Figs. 4a, 4b, 4c. Views from the side, from above, and from below, of a brain of Scyllium canicula during stage P. In the view from the side the eye (op) has not been removed.
The bilobed appearance both of the mid-brain and cerebellum should be noticed.
Fig. 5. Longitudinal section of a brain of Scyllium canicula during stage P. Zeiss a,ocul.2.
There should be noticed (1) the increase in the flexure of the brain accompanying a rectification of the cranial axis; (2) the elongated pineal gland, and (3) the structure of the optic thalamus.
Figs. 6a, 6b, 6c. Views from the side, from above, and from below, of a brain of Scyllium stellare during a slightly later stage than Q.
Figs. 7aand 7b. Two longitudinal sections through the brain of a Scyllium embryo during stage Q. Zeiss a,ocul.2.
7acuts the hind part of the brain nearly through the middle line; while 7bcuts the cerebral hemispheres and pineal gland through the middle.
In 7athe infundibulum (1), cerebellum (2), the passage of the restiform tracts (rt) into the cerebellum (3), and the rudiments of the tela vasculosa (4) are shewn. In 7bthe septum between the two lobes of the cerebral hemispheres (1), the pineal gland (2), and the relations of the optic thalami (3) are shewn.
Figs. 8a, 8b, 8c, 8d. Four transverse sections of the brain of an embryo slightly older than Q. Zeiss a,ocul.1.
8apasses through the cerebral hemispheres at their junction with the olfactory lobes. On the right side is seen the olfactory nerve coming off from the olfactory lobe. At the dorsal side of the hemispheres is seen the pineal gland (pn).
8bpasses through the mid-brain now slightly bilobed, and the opening into the infundibulum (in). At the base of the section are seen the optic nerves and their chiasma.
8cpasses through the opening from the ventricle of the mid-brain into that of the cerebellum. Below the optic lobes is seen the infundibulum with the rudiments of the sacci vasculosi.
8dpasses through the front end of the medulla, and shews the roots of the seventh pair of nerves, and the overlapping of the medulla by the cerebellum.
Plate 17. (X.p.397.)
Complete List of Reference Letters.
vii.a.Anterior branch of seventh nerve.ar.Anterior root of spinal nerve.auv.Auditory vesicle.cer.Cerebrum.ch.Notochord.ch.Epithelial layer of choroid membrane.gl.Glossopharyngeal nerve.vii.hy.Hyoid branch of seventh nerve.hym.Hyaloid membrane.ll.Lateral line.v.mn.Ramus mandibularis of fifth nerve.vii.mn.Mandibular (spiracular) branch of seventh nerve.v.mx.Ramus maxillæ superioris of fifth nerve.nl.Nervus lateralis.ol.Olfactory pit.op.Eye.v.opth.Ramus ophthalmicus of fifth nerve.pch.Parachordal cartilage.pfal.Processus falciformis.pp.Head cavity.pr.Posterior root of spinal nerve.rt.Retina.sp.Spiracle. v. Fifth nerve.vii. Seventh nerve.vc.Visceral cleft.vg.Vagus nerve.vg.br.Branchial branch of vagus.vgcom.Commissure uniting the roots of the vagus, and continuous with commissure uniting the posterior roots of the spinal nerves.vgr.Roots of vagus nerves in the brain.vgin.Intestinal branch of vagus.vh.Vitreous humour.
Fig. 1. Diagram of cranial nerves at stage L.
A description of the part of this referring to the vagus and glossopharyngeal nerves is given atp.426. It should be noticed that there are only five strands indicated as springing from the spinal cord to form the vagus and glossopharyngeal nerves. It is however probable that there are even from the first a greater number of strands than this.
Fig. 2. Section through the hinder part of the medulla oblongata, stage between K and L. Zeiss A,ocul.2.
It shews (1) the vagus commissure with branches on one side from the medulla: (2) the intestinal branch of the vagus giving off a nerve to the lateral line.
Fig. 3. Longitudinal and vertical section through the head of a Scyllium embryo of stage L. Zeiss a,ocul.2.
It shews the course of the anterior branch of the seventh nerve (vii.); especially with relation to the ophthalmic branch of the fifth nerve (v.oth).
Figs. 4aand 4b. Two horizontal and longitudinal sections through the head of a Scyllium embryo belonging to stage O. Zeiss a,ocul.2.
4ais the most dorsal of the two sections, and shews the course of the anterior branch of the seventh nerve above the eye.
4bis a slightly more ventral section, and shews the course of the fifth nerve.
Fig. 5. Longitudinal and horizontal section through the hind-brain at stage O, shewing the roots of the vagus and glossopharyngeal nerves in the brain. Zeiss B,ocul.2.
There appears to be one root in the brain for the glossopharyngeal, and at least six for the vagus. The fibres from the roots divide in many cases into two bundles before leaving the brain. Swellings of the brain towards the interior of the fourth ventricle are in connection with the first five roots of the vagus, and the glossopharyngeal root; and a swelling is also intercalated between the first vagus root and the glossopharyngeal root.
Fig. 6. Horizontal section through a part of the choroid slit at stage P. Zeiss B,ocul.2.
The figure shews (1) the rudimentary processus falciformis (pfal) giving origin to the vitreous humour; and (2) the hyaloid membrane (hym) which is seen to adhere to the retina, and not to the vitreous humour or processus falciformis.
EXPLANATION OFPLATE 18. (X.p.446.)
Complete List of Reference Letters.
Nervous System.
ar.Anterior root of spinal nerve.nc.Neural canal.pr.Posterior root of spinal nerve.spn.Spinal nerve.syg.Sympathetic ganglion.
Alimentary Canal.
al.Alimentary canal.alv.Caudal vesicle of the postanal gut.clal.Cloacal section of alimentary canal.du.Duodenum.hpd.Ductus choledochus.pan.pancreas.sœs.Solid œsophagus.spv.Intestine with rudiment of spiral valve.umc.Umbilical canal.
General.
ao.Dorsal aorta.aur.Auricle of heart.cav.Cardinal vein.ch.Notochord.eppp.Epithelial lining of the body-cavity.ir.Interrenal body.me.Mesentery.mp.Muscle-plate.mpl.Muscle-plate sending a prolongation into the limb.po.Primitive ovum.pp.Body-cavity.sd.Segmental duct.st.Segmental tube.ts.Tail swelling.vcau.Caudal vein.x.Subnotochordal rod.
Fig. 1. Transverse section through the anterior abdominal region of an embryo of a stage between K and L. Zeiss B,ocul.2. Reduced one-third.
The section illustrates the junction of a sympathetic ganglion with a spinal nerve and the sprouting of the muscle-plates into the limbs (mpl).
Fig. 2. Transverse section through the abdominal region of an embryo belonging to stage L. Zeiss B,ocul.2. Reduced one-third.
The section illustrates the junction of a sympathetic ganglion with a spinal nerve, and also the commencing formation of a branch from the aorta (still solid) which will pass through the sympathetic ganglion, and forms the first sign of the conversion of part of a sympathetic ganglion into one of the suprarenal bodies.
Fig. 3. Longitudinal and vertical section of an embryo of a stage between L and M, shewing the successive junctions of the spinal nerves and sympathetic ganglia.
Fig. 4. Section through the solid œsophagus during stage L. Zeiss A,ocul.1. The section is taken through the region of the heart, so that the cavity of the auricle (aur) lies immediately below the œsophagus.
Fig. 5. Optical section of the tail of an embryo between stages I and K, shewing the junction between the neural and alimentary canals.
Fig. 6. Four sections through the caudal region of an embryo belonging to stage K, shewing the condition of the postanal section of the alimentary tract. Zeiss A,ocul.2. An explanation of these figures is given onp.449.
Fig. 7. Section through the interrenal body of a Scyllium embryo belonging to stage Q. Zeiss C,ocul.2.
Fig. 8. Portion of a section of the interrenal body of an adult Scyllium. Zeiss C,ocul.2.
EXPLANATION OFPLATE 19. (X.p.460)
Complete List of Reference Letters.
Nervous System.
n.Nerve.spn.Spinal nerve.syg.Sympathetic ganglion.
Alimentary Canal.
cl.Cloaca.incl.Cloacal involution.œep.Œsophageal epithelium.pan.Pancreas.th.Thyroid body.
General.
abp.Abdominal pocket (pore).aur.Auricle.cav.Cardinal vein.cauv.Caudal vein.ly.Lymphoid tissue.mm.Muscles.od.Oviduct.pc.Pericardium.pp.body-cavity.sr.Suprarenal body.u.Ureter.vao.Ventral aorta (anterior continuation of bulbus arteriosus).ven.Ventricle.wd.Wolffian duct.
Figs. 1a, 1b, 1c. Three sections through the cloacal region of an embryo belonging to stage O. 1ais the anterior of the three sections. Zeiss A,ocul.2. Reduced one-third.
1ashews the cloacal involution at its deepest part abutting on the cloacal section of the alimentary tract.
1bis a section through a point somewhat behind this close to the opening of the Wolffian ducts into the cloaca.
1cshews the opening to the exterior in the posterior part of the cloaca, and also the rudiments of the two abdominal pockets (abp).
Fig. 2. Section through the cloacal region of an embryo belonging to stage P. Zeiss A,ocul.2.
The figure shews the solid anterior extremity of the cloacal involution.
Fig. 3. Longitudinal vertical section through the thyroid body in a stage between O and P. Zeiss a a,ocul.1.
The figure shews the solid thyroid body (th) connected in front with throat, and terminating below the bulbus arteriosus.
Fig. 4. Pancreas (pan) and adjoining part of the alimentary tract in longitudinal section, from an embryo between stages L and M. Zeiss A,ocul.2.
Fig. 5. Portion of liver network of stage L. Zeiss C,ocul.2. The section is intended to illustrate the fact that the tubules or cylinders of which the liver is composed are hollow and not solid. Between the liver tubules are seen blood spaces with distinct walls, and blood corpuscles in their interior.
Fig. 6. Section through part of one of the suprarenal bodies of an adult Scyllium hardened in chromic acid. Zeiss C,ocul.2. The section shews the columnar cells forming the cortex and the more polygonal cells of the medulla.
Fig. 7. Transverse section through the anterior suprarenal body of an adult Scyllium. Zeiss B,ocul.2. Reduced one-third. The tissue of the suprarenal body has not been filled in, but only the sympathetic ganglion cells which are seen to be irregularly scattered through the substance of the body. The entrance of the nerve (n) is shewn, and indications are given of the distribution of the nerve-fibres.
Fig. 8. Section through the sympathetic ganglion of a Scyllium embryo between stages M and N, shewing the connecting trunk between the suprarenal body and the spinal nerve (spn), and the appearance of an indication in the ganglion of a portion more directly connected with the nerve. Zeiss D,ocul.2.
Fig. 9. Section through one of the anterior sympathetic ganglia of an embryo of stage Q, shewing its division into a true ganglionic portion (syg), and a suprarenal body (sr). Zeiss C,ocul.2.
EXPLANATION OFPLATES 20AND21. (X.p.479.)
Complete List of Reference Letters.
amg.Accessory Malpighian body.cav.Cardinal vein.ge.Germinal epithelium.k.True kidney.l.c.Longitudinal canal of the Wolffian body connected with vasa efferentia.mg.Malpighian body.nt.Network and central canal at the base of the testis.o.External aperture of urinal cloaca.od.Oviduct or Müllerian duct of the female.od´.Müllerian duct of the male.ou.Openings of ureters in Wolffian duct in the female (fig. 3).pmg.Primary Malpighian body.px.Growth from vesicle at the end of a segmental tube to join the collecting tube of the preceding segment.rst.Rudimentary segmental tube.ru.Ureter commencing to be formed.sb.Seminal bladder.sd.Segmental duct.st.Segmental tube.sto.Opening of segmental tube into body-cavity.sur.Suprarenal body.t.Testis.u.Ureters.ve.Vas efferens.wb.Wolffian body.wd.Wolffian duct.
Plate 20.
Fig. 1. Diagrammatic representation of excretory organs on one side of a male Scyllium canicula, natural size.
Fig. 2. Diagrammatic representation of the kidney proper on one side of a female Scyllium canicula, natural size, shewing the ducts of the kidney and the dilated portion of the Wolffian duct.
Fig. 3. Opening of the ureters into the Wolffian duct of a female Scyllium canicula. The figure represents the Wolffian ducts (wd) with ventral portion removed so as to expose their inner surface, and shews the junction of the two W. ducts to form the common urinal cloaca, the single external opening of this (o), and openings of ureters into one Wolffian duct (ou).
Fig. 4. Anterior extremity of Wolffian body of a young male Scyllium canicula shewing the vasa efferentia and their connection with the kidneys and the testis. The vasa efferentia and longitudinal canal are coloured to render them distinct. They are intended to be continuous with the uncoloured coils of the Wolffian body, though this connection has not been very successfully rendered by the artist.
Fig. 5. Part of the Wolffian body of a nearly ripe male embryo of Scyllium canicula as a transparent object. Zeiss a a,ocul.3. The figure shews two segmental tubes opening into the body-cavity and connected with a primary Malpighian body, and also, by a fibrous connection, with a secondary Malpighian body of the preceding segment. It also shews one segmental tube (rst) imperfectly connected with the accessory Malpighian body of the preceding segment of the kidney. The coils of the kidney are represented somewhat diagrammatically.
Fig. 6. Vasa efferentia of a male embryo of Scyllium canicula eight centimetres in length. Zeiss a a,ocul.2.
There are seen to be at the least six and possibly seven distinct vasa going to as many segments of the Wolffian body and connected with a longitudinal canal in the base of the testis. They were probably also connected with a longitudinal canal in the Wolffian body, but this could not be clearly made out.
Fig. 7. The anterior four vasa efferentia of a nearly ripe embryo. Connected with the foremost one is seen a body which looks like the remnant of a segmental tube and its opening (rst?).
Fig. 8. Testis and anterior part of Wolffian body of an embryo of Squatina vulgaris.
The figure is intended to illustrate the arrangement of the vasa efferentia. There are five of these connected with a longitudinal canal in the base of the testis, and with another longitudinal canal in the Wolffian body. From the second longitudinal canal there pass off four ducts to as many Malpighian bodies. Through the Malpighian bodies these ducts are continuous with the several coils of the Wolffian body, and so eventually with the Wolffian duct. Close to the hindermost vas efferens is seen a body which resembles a rudimentary segmental tube (rst?).
Plate 21.
Figs. 1A, 1B, 1C, 1D. Four sections from a female Scyllium canicula of a stage between M and N through the part where the segmental duct becomes split into Wolffian duct and oviduct. Zeiss B,ocul.2. 1A is the foremost section.
The sections shew that the oviduct arises as a thickening on the under surface of the segmental duct into which at the utmost a very narrow prolongation of the lumen of the segmental duct is carried. The small size of the lumen of the Wolffian duct in the foremost section is due to the section passing through nearly its anterior blind extremity.
Fig. 2. Section close to the junction of the Wolffian duct and oviduct in a female embryo of Scyllium canicula belonging to stage N. Zeiss B,ocul.2.
The section represented shews that in some instances the formation of the oviduct and Wolffian duct is accompanied by a division of the lumen of the segmental duct into two not very unequal parts.
Figs. 3A, 3B, 3C. Three sections illustrating the formation of a ureter in a female embryo belonging to stage N. Zeiss B,ocul.2.
3A is the foremost section.
The figures shew that the lumen of the developing ureter is enclosed in front by an independent wall (fig. 3A), but that further back the lumen is partly shut in by the subjacent Wolffian duct, while behind no lumen is present, but the ureter ends as a solid knob of cells without an opening into the Wolffian duct.
Fig. 4. Section through the ureters of the same embryo as fig. 3, but nearer the cloaca. Zeiss B,ocul.2.
The figure shews the appearance of a transverse section through the wall of cells above the Wolffian duct formed by the overlapping ureters, the lumens of which appear as perforations in it. It should be compared with fig. 9A, which represents a longitudinal section through a similar wall of cells.
Fig. 5. Section through the ureters, the Wolffian duct and the oviduct of a female embryo of Scy. canicula belonging to stage P. Zeiss B,ocul.2.
Fig. 6. Section of part of the Wolffian body of a male embryo of Scyllium canicula belonging to stage O. Zeiss B,ocul.2.
The section illustrates (1) the formation of a Malpighian body (mg) from the dilatation at the end of a segmental tube, (2) the appearance of the rudiment of the Müllerian duct in the male (od´).
Figs. 7a, 7b. Two longitudinal and vertical sections through part of the kidney of an embryo between stages L and M. Zeiss B,ocul.2.
7aillustrates the parts of a single segment of the Wolffian body at this stage,videp.491. The segmental tube and opening are not in the plane of the section, but the dilated vesicle is shewn into which the segmental tube opens.
7bis taken from the region of the kidney proper. To the right is seen the opening of a segmental tube into the body-cavity, and in the segment to the left the commencing formation of a ureter,videp.502.
Fig. 8. Longitudinal and vertical section through the posterior part of the kidney proper of an embryo of Scyllium canicula at a stage between N and O. Zeiss A,ocul.2.
The section shews the nearly completed ureters, developing Malpighian bodies,&c.
Fig. 9. Longitudinal and vertical section through the anterior part of the kidney proper of the same embryo as fig. 8. Zeiss A,ocul.2.
The figure illustrates the mode of growth of the developing ureters.
9A. More highly magnified portion of the same section as fig. 9.
Compare with transverse section fig. 4.
Fig. 10. Longitudinal and vertical section through part of the Wolffian body of an embryo of Scyllium canicula at a stage between O and P.
The section contains two examples of the budding out of the vesicle of a segmental tube to form a Malpighian body in its own segment and to unite with the tubulus of the preceding segment close to its opening into the Wolffian duct.
EXPLANATION OF THEPLATES 22AND23[3]. (VIII. p. 168.)
Plate 22.
Fig. A. Section through the dorsal region of an embryo ofScyllium stellare, with the rudiments of two visceral clefts. The section illustrates the general features at a period anterior to the appearance of the posterior nerve-roots.
nc.neural canal.mp.muscle-plate.ch.notochord.x.subnotochordal rod.ao.rudiment of dorsal aorta.so.somatopleure.sp.splanchnopleure.al.alimentary tract. All the parts of the section except the spinal cord are drawn somewhat diagrammatically.
Figs.BI, BII, BIII. Three sections of aPristiurus-embryo.BIis through the heart,BIIthrough the anterior part of the dorsal region, andBIIIthrough a point slightly behind this. Drawn with a camera. (ZeissCCocul.2.)
InBIIIthere is visible a slight proliferation of cells from the dorsal summit of the neural canal.
InBIIthis proliferation definitely constitutes two club-shaped masses of cells (pr), both attached to the dorsal summit of the neural canal. The masses are the rudiments of the posterior nerve-roots.
InBIthe rudiments of the posterior roots are of considerable length.
pr.rudiment of posterior roots.nc.neural canal.mp.muscle-plate.ch.notochord.x.subnotochordal rod.ao.dorsal aorta.so.somatopleure.sp.splanchnopleure.al.alimentary canal.ht.heart.
Fig. C. Section from aPristiurus-embryo, slightly older than B. Camera. (ZeissCCocul.2.) The embryo from which this figure was taken was slightly distorted in the process of removal from the blastoderm.
vr.rudiment of vertebral body. Other reference letters as in previous figures.
Fig.D a.Section through the dorsal region of aTorpedo-embryo with three visceral clefts. (ZeissCCocul.2.) The section shews the formation of the dorsal nerve-rudiments (pr) and of a ventral anterior nerve-rudiment (ar), which at this early stage is not distinctly cellular.
ar.rudiment of an anterior nerve-root.y.cells left behind on the separation of the external skin from the spinal cord.c.connective-tissue cells springing from the summit of the muscle-plates. Other reference letters as above.
Fig.D b.Section from dorsal region of aTorpedo-embryo somewhat older thanD a.Camera. (ZeissCCocul.2.) The posterior nerve-rudiment is considerably longer than in fig. Da, and its pedicle of attachment to the spinal cord is thinner. The anterior nerve-rudiment, of which only the edge is present in the section, is distinctly cellular.
m.mesoblast growing up from vertebral rudiment.sd.segmental duct.
Fig.D c.Section from a still olderTorpedo-embryo. Camera. (ZeissCCocul.2.) The connective-tissue cells are omitted. The rudiment of the ganglion (g) on the posterior root has appeared. The rudiment of the posterior nerve is much longer than before, and its junction with the spinal cord is difficult to detect. The anterior root is now an elongated cellular structure.
g.ganglion.
Fig.D d.Longitudinal and vertical section through aTorpedo-embryo of the same age as D c.
The section shews the commissures (x) uniting the posterior roots.
Fig.E a.Section of aPristiurus-embryo belonging to the second stage. Camera. (ZeissCCocul.2.) The section shews the constriction of the pedicle which attaches the posterior nerve-rudiments to the spinal cord.
pr.rudiment of posterior nerve-root.nc.neural canal.mp.muscle-plate.vr.vertebral rudiment.sd.segmental duct.ch.notochord.so.somatopleure.sp.splanchnopleure.ao.aorta.al.alimentary canal.
Fig.E b.Section of aPristiurus-embryo slightly older thanEa.Camera. (ZeissCCocul.2.) The section shews the formation of the anterior nerve-root (ar).
ar.rudiment of the anterior nerve-root.
Fig. F. Section of aPristiurus-embryo with the rudiments of five visceral clefts. Camera. (ZeissCCocul.2.)
The rudiment of the posterior root is seen surrounded by connective-tissue, from which it cannot easily be distinguished. The artist has not been very successful in rendering this figure.
Figs.G1, G2, G3. Three longitudinal and horizontal sections of an embryo somewhat older than F. The embryo from which these sections were taken was hardened in osmic acid, but the sections have been represented without tinting.G1is most dorsal of the three sections. Camera. (ZeissCCocul.1.)
nc.neural canal.sp.c.spinal cord.pr.rudiment of posterior root.ar.rudiment of anterior root.mp.muscle-plate.c.connective-tissue cells.ch.notochord.
Plate 23.
Fig. HI.Section through the dorsal region of aPristiurus-embryo in which the rudimentary external gills are present as very small knobs. Camera. (ZeissCCocul.2.)
The section shews the commencing differentiation of the posterior nerve-rudiment into root (pr), ganglion (sp.g), and nerve (n), and also the attachment of the nerve-root to the spinal cord (x). The variations in the size and shape of the cells in the different parts of the nerve-rudiment are completely lost in the figure.
pr.posterior nerve-root.sp.g.ganglion of posterior root.n.nerve of posterior root.x.attachment of posterior root to spinal cord.w.white matter of spinal cord.i.mesoblastic investment to the spinal cord.
Fig. HII.Section through the same embryo asHI. (ZeissCCocul.1.)
The section contains an anterior root, which takes its origin at a point opposite the interval between two posterior roots.
The white matter has not been very satisfactorily represented by the artist.
Figs.II, III. Two sections of aPristiurus-embryo somewhat older than H. Camera. (ZeissCCocul.1.)
The connective-tissue cells are omitted.
Figs.I a, I b, I c. Three isolated cells from the ganglion of one of the posterior roots of the same embryo.
Figs.KI, KII. Two horizontal longitudinal sections through an embryo in which the external gills have just appeared.KIis the most dorsal of the two sections. Camera. (ZeissCCocul.1.)
The sections shew the relative positions of the anterior and posterior roots at different levels.
pr.posterior nerve-rudiment.ar.anterior nerve-rudiment.sp.c.spinal cord.n.c.neural canal.mp.muscle-plate.mp´.first-formed muscles.
Fig. L. Longitudinal and vertical section through the trunk of aScyllium-embryo after the external gills have attained their full development. Camera. (ZeissCCocul.1.)
The embryo was hardened in a mixture of chromic acid and osmic acid.
The section shews the commissures which dorsally unite the posterior roots, and also the junction of the anterior and posterior roots. The commissures are unfortunately not represented in the figure with great accuracy; their outlines are in nature perfectly regular, and not, as in the figure, notched at the junctions of the cells composing them. Their cells are apparently more or less completely fused, and certainly not nearly so clearly marked as in the figure. The commissures stain very deeply with the mixture of osmic and chromic acid, and form one of the most conspicuous features in successful longitudinal sections of embryos so hardened. In sections hardened with chromic acid only they cannot be seen with the same facility.
sp.c.spinal cord.gr.grey matter.w.white matter.ar.anterior root.pr.posterior root.x.commissure uniting the posterior roots.
Figs.MI, MII. Two sections through the head of the same embryo as fig. B.MI, the foremost of the two, passes through the anterior part of the thickening of epiblast, which becomes involuted as the auditory vesicle. It contains the rudiment of the seventh nerve, VII. Camera. (ZeissCCocul.2.)
VII.rudiment of seventh nerve.au.thickening of external epiblast, which becomes involuted as the auditory vesicle.n.c.neural canal.ch.notochord.pp.body-cavity in the head.so.somatopleure.sp.splanchnopleure.al.throat exhibiting an outgrowth to form the first visceral cleft.
[3]The figures on these Plates give a fair general idea of the appearance presented by the developing spinal nerves; but the finer details of the original drawings have in several cases become lost in the process of copying.
The figures which are tinted represent sections of embryos hardened in osmic acid; those without colour sections of embryos hardened in chromic acid.
EXPLANATION OF THE PLATES24,25,26. (XII.p.549.)
Plate 24.
List of Reference Letters.
dn.Modified nucleus of primitive ovum.do.Permanent ovum in the act of being formed.dv.Developing blood-vessels.dyk.Developing yolk.ep.Non-ovarian epithelium of ovarian ridge.fe.Follicular epithelium.gv.Germinal vesicle.lstr.Lymphatic region of stroma.nn.Nests of nuclei of ovarian region.o.Permanent ovum.ovr.Ovarian portion of ovarian ridge.po.Primitive ovum.pse.Pseudo-epithelium of ovarian ridge.str.Stroma ingrowths into ovarian epithelium.v.Blood-vessel.vstr.Vascular region of stroma adjoining ovarian ridge.vt.Vitelline membrane.x.Modified nucleus.yk.Yolk.zn.Zona radiata.
Fig. 1. Transverse section of the ovarian ridge of an embryo ofScy.canicula, belonging to stage P, shewing the ovarian region with thickened epithelium and numerous primitive ova. Zeiss C,ocul.2.Picric acid.
Fig. 2. Transverse section of the ovarian ridge of an embryo ofScyllium canicula, considerably older than stage Q. Zeiss C,ocul.2.Picric acid.Several nests, some with distinct ova, and others with the ova fused together, are present in the section (n.n.), and several examples of modified nuclei in still distinct ova are also represented. One of these is markedx. The stroma of the ovarian ridge is exceptionally scanty.
Fig. 3. Transverse section through part of the ovarian ridge, including the ovarian region of an almost ripe embryo ofScyllium canicula. Zeiss C,ocul.2.Picric acid.Nuclear nests (n.n.), developing ova (d.o.), and ova (o.), with completely formed follicular epithelium, are now present. The ovarian region is still well separated from the subjacent stroma, and does not appear to contain any cells except those of the original germinal epithelium.
Fig. 4. Section through ovarian ridge of the same embryo as fig. 3, to illustrate the relation of the stroma (str.) and ovarian region. Zeissa a,ocul.2.Picric acid.
Fig. 5. Section through the ovarian ridge of an embryo ofScyllium canicula, 10cm.long, in which the ovary was slightly less advanced than in fig. 3. To illustrate the relation of the ovarian epithelium to the subjacent vascular stroma. Zeiss A,ocul.2.Osmic acid.y.points to a small separated portion of the germinal epithelium.
Fig. 6. Section through the ovarian ridge of an embryo ofScyllium canicula, slightly older than fig. 5. To illustrate the relation of the ovarian epithelium to the subjacent vascular stroma. Zeiss A,ocul.2.Osmic acid.
Fig. 7. More highly magnified portion of the same ovary as fig. 6. To illustrate the same points. Zeiss C,ocul.2.Osmic acid.
Fig. 8. Section through the ovarian region (close to one extremity, where it is very small) from a young female ofScy.canicula. Zeiss C,ocul.2.Picric acid.It shews the vascular ingrowths amongst the original epithelial cells of the ovarian region.
Fig. 9. Section through the ovarian region of the same embryo as fig. 8, at its point of maximum development. Zeiss A,ocul.2.Picric acid.
Fig. 10. Section through superficial part of the ovary of an embryo, shewing the pseudo-epithelium; the cells of which are provided with tails prolonged into the general tissue of the ovary. Atf.e.is seen a surface view of the follicular epithelium of an ovum. Zeiss C,ocul.2.Picric acid.
Fig. 11. Section through part of an ovary ofScyllium caniculaof stage Q, with three primitive ova, the most superficial one containing a modified nucleus.
Fig. 12. Section through part of an ovary of an example ofScyllium canicula, 8cm.long. The section passes through a nest of ova with modified nuclei, in which the outlines of the individual ova are quite distinct. Zeiss E,ocul.2.Picric acid.
Fig. 13. Section through part of ovary of the same embryo as in fig. 5. The section passes through a nest of nuclei, with at the least two developing ova, and also through one already formed permanent ovum. Zeiss E,ocul.2.Osmic acid.
Figs. 14, 15, 16, 17, 18 [Figs. 17 and 18 are onPl.25]. Sections through parts of the ovary of the same embryo as fig. 3, with nests of nuclei and a permanent ova in the act of formation. Fig. 14 is drawn with Zeiss D D,ocul.2. Figs. 15, 16, 17, with Zeiss E,ocul.2.Picric acid.
Plate 25.
List of Reference Letters.
do.Permanent ovum in the act of being formed.dyk.Developing yolk.fe.Follicular epithelium.fe´.Secondary follicular epithelium.gv.Germinal vesicle.nn.Nests of nuclei of ovarian region.o.Permanent ovum.pse.Pseudo-epithelium.str.Stroma ingrowths into ovarian epithelium.vt.Vitelline membrane.x.Modified nucleus.yk.Yolk (vitellus).zn.Zona radiata.
[Figs. 17 and 18.Videdescription of Plate 24.].
Fig. 19. Two nuclei from a nest which appear to be in the act of division. From ovary of the same embryo as fig. 3.
Fig. 20. Section through part of an ovary of the same embryo as fig. 6, containing a nest of nuclei. Zeiss F,ocul.2.Osmic acid.
Fig. 21. Ovum from the ovary of a half-grown female, containing isolated deeply stained patches of developing yolk granules. Zeiss B,ocul.2.Picric acid.
Fig. 22. Section through a small part of the ovum of an immature female ofScyllium canicula, to shew the constitution of the yolk, the follicular epithelium, and the egg membranes. Zeiss E,ocul.2.Chromic acid.
Fig. 23. Section through part of the periphery of a nearly ripe ovum ofScy.canicula. Zeiss C,ocul.2. It shews the remnant of the vitelline membrane (v.t.) separating the columnar but delicate cells of the follicular epithelium (f.e.) from the yolk (yk.). In the yolk are seen yolk-spherules in a protoplasmic network. The transverse markings in the yolk-spherules have been made oblique by the artist.