Illustration: Figure 374Fig. 374. Diagram of the chief venous trunks of Man.(From Gegenbaur.)cs.vena cava superior;s.subclavian vein;ji.internal jugular;je.external jugular;az.azygos vein;ha.hemiazygos vein;c.dotted line shewing previous position of cardinal veins;ci.vena cava inferior;r.renal veins;il.iliac;hy.hypogastric veins;h.hepatic veins.The dotted lines shew the position of embryonic vessels aborted in the adult.
Fig. 374. Diagram of the chief venous trunks of Man.(From Gegenbaur.)cs.vena cava superior;s.subclavian vein;ji.internal jugular;je.external jugular;az.azygos vein;ha.hemiazygos vein;c.dotted line shewing previous position of cardinal veins;ci.vena cava inferior;r.renal veins;il.iliac;hy.hypogastric veins;h.hepatic veins.The dotted lines shew the position of embryonic vessels aborted in the adult.
At a later period a pair of trunks is established bringing the blood from the posterior part of the cardinal veins and the crural veins directly into the vena cava inferior (fig. 374,il). These vessels, whose development has not been adequately investigated, form the common iliac veins, while the posterior ends of the cardinal veins which join them become the hypogastric veins (fig. 374,hy). Owing to the development of the common iliac veins there is no renal portal system like that of the Reptilia and Amphibia.
Posterior vertebral veins, similar to those of Reptilia and Birds, are established in connection with the intercostal and lumbar veins, and unite anteriorly with the front part of the posterior cardinal veins (fig. 373A)[238].
On the formation of the posterior vertebral veins, and as the inferior vena cava becomes more important, the middle part of the posterior cardinals becomes completely aborted (fig. 374,c), the anterior and posterior parts still persisting, the former as the continuations of the posterior vertebrals into the anterior vena cava (az), the latter as the hypogastric veins (hy).
Though in a few Mammalia both the posterior vertebrals persist, a transverse connection is usually established between them, and the one (the right) becoming the more important constitutes the azygos vein (fig. 374,az), the persisting part of the left forming the hemiazygos vein (ha).
The remainder of the venous system is formed in the embryo of the vitelline and allantoic veins, the former being eventually joined by the mesenteric vein so as to constitute the portal vein.
The vitelline vein is the first part of this system established, and divides near the heart into two veins bringing back the blood from the yolk-sack (umbilical vesicle). The right vein soon however aborts.
The allantoic (anterior abdominal) veins are originally paired. They are developed very early, and at first course along the still widely open somatic walls of the body, and fall into the single vitelline trunk in front. The right allantoic vein disappears before long, and the common trunk formed by the junction of the vitelline and allantoic veins becomes considerably elongated. This trunk is soon enveloped by the liver.
The succeeding changes have been somewhat differently described by Kölliker and Rathke. According to the former the common trunk of the allantoic and vitelline veins in its passage through the liver gives off branches to the liver, and also receives branches from this organ near its anterior exit. The main trunk is however never completely aborted, as in the embryos of other types, but remains as the ductus venosus Arantii.
With the development of the placenta the allantoic vein becomes the main source of the ductus venosus, and the vitelline or portal vein, as it may perhaps be now conveniently called, ceases to join it directly, but falls into one of its branches in the liver.
The vena cava inferior joins the continuation of the ductus venosus in front of the liver, and, as it becomes more important, it receives directly the hepatic veins which originally brought back blood into the ductus venosus. The ductus venosus becomes moreover merely a small branch of the vena cava.
At the close of fœtal life the allantoic vein becomes obliterated up to its place of entrance into the liver; the ductus venosus becomes a solid cord—the so-called round ligament—and the whole of the venous blood is brought to the liver by the portal vein[239].
Owing to the allantoic (anterior abdominal) vein having merely a fœtal existence an anastomosis between the iliac veins and the portal system by means of the anterior abdominal vein is not established.
Bibliographyof the Venous System.
(498)J. Marshall. “On the development of the great anterior veins.”Phil. Trans., 1859.(499)H. Rathke. “Ueb. d. Bildung d. Pfortader u. d. Lebervenen b. Säugethieren.”Meckel’s Archiv, 1830.(500)H. Rathke. “Ueb. d. Bau u. d. Entwick. d. Venensystems d. Wirbelthiere.”Bericht. üb. d. naturh. Seminar. d. Univ. Königsberg, 1838.Videalso Von Baer (No.291), Götte (No.296), Kölliker (No.298), and Rathke (Nos.299,300, and301).
Lymphatic System.
The lymphatic system arises from spaces in the general parenchyma of the body, independent in their origin of the true body cavity, though communicating both with this cavity and with the vascular system.
In all the true Vertebrata certain parts of the system form definite trunks communicating with the venous system; and in the higher types the walls of the main lymphatic trunks become quite distinct.
But little is known with reference to the ontogeny of the lymphatic vessels, but they originate late in larval life, and have at first the form of simple intercellular spaces.
The lymphatic glands appear to originate from lymphatic plexuses, the cells of which produce lymph corpuscles. It is only in Birds and Mammals, and especially in the latter, that the lymphatic glands form definite structures.
The Spleen.The spleen, from its structure, must be classed with the lymphatic glands, though it has definite relations to the vascular system. It is developed in the mesoblast of the mesogastrium, usually about the same time and in close connection with the pancreas.
According to Müller and Peremeschko the mass of mesoblast which forms the spleen becomes early separated by a groove on the one side from the pancreas and on the other from the mesentery. Some of its cells become elongated, and send out processes which uniting with like processes from other cells form the trabecular system. From the remainder of the tissue are derived the cells of the spleen pulp, which frequently contain more than one nucleus. Especial accumulations of these cells take place at a later period to form the so-called Malpighian corpuscles of the spleen.
Bibliographyof Spleen.
(501)W. Müller. “The Spleen.”Stricker’s Histology.(502)Peremeschko. “Ueb. d. Entwick. d. Milz.”Sitz. d. Wien. Akad. Wiss.,Vol.LVI. 1867.
Suprarenal bodies.
In Elasmobranch Fishes two distinct sets of structures are found, both of which have been called suprarenal bodies. As shewn in the sequel both of these structures probably unite in the higher types to form the suprarenal bodies.
One of them consists of a series of paired bodies, situated on the branches of the dorsal aorta, segmentally arranged, and forming a chain extending from close behind the heart to the hinder end of the body cavity. Each body is formed of a series of lobes, and exhibits a well-marked distinction into a cortical layer of columnar cells, and a medullary substance formed of irregular polygonal cells. As first shewn by Leydig, they areclosely connected with the sympathetic ganglia, and usually contain numerous ganglion cells distributed amongst the proper cells of the body.
The second body consists of an unpaired column of cells placed between the dorsal aorta and unpaired caudal vein, and bounded on each side by the posterior parts of the kidney. I propose to call it theinterrenal body. In front it overlaps the paired suprarenal bodies, but does not unite with them. It is formed of a series of well-marked lobules, etc. In the fresh state Leydig (No.506) finds that “fat molecules form the chief mass of the body, and one finds freely imbedded in them clear vesicular nuclei.” As may easily be made out from hardened specimens it is invested by a tunica propria, which gives off septa dividing it into well-marked areas filled with polygonal cells. These cells constitute the true parenchyma of the body. By the ordinary methods of hardening, the oil globules, with which they are filled in the fresh state, completely disappear.
The paired suprarenal bodies (Balfour,No.292,pp.242-244) are developed from the sympathetic ganglia. These ganglia, shewn in an early stage infig. 380,sy.g, become gradually divided into a ganglionic part and a glandular part. The former constitutes the sympathetic ganglia of the adult; the latter the true paired suprarenal bodies. The interrenal body is however developed (Balfour,No.292,pp.245-247) from indifferent mesoblast cells between the two kidneys, in the same situation as in the adult.
The development of the suprarenal bodies in the Amniota has been most fully studied by Braun (No.503) in the Reptilia.
In Lacertilia they consist of a pair of elongated yellowish bodies, placed between the vena renalis revehens and the generative glands.
They are formed of two constituents,viz.(1) masses of brown cells placed on the dorsal side of the organ, which stain deeply with chromic acid, like certain of the cells of the suprarenals of Mammalia, and (2) irregular cords, in part provided with a lumen, filled with fat-like globules[240], amongst which are nuclei. On treatment with chromic acid the fat globules disappear, and the cords break up into bodies resembling columnar cells.
The dorsal masses of brown cells are developed from the sympathetic ganglia in the same way as the paired suprarenal bodies of the Elasmobranchii, while the cords filled with fat-like globules are formed of indifferent mesoblast cells as a thickening in the lateral walls of the inferior vena cava, and the cardinal veins continuous with it. The observations of Brunn (No.504) on the Chick, and Kölliker (No.298,pp.953-955) on the Mammal, add but little to those of Braun. They shew that the greater part of the gland (the cortical substance) in these two types is derived from the mesoblast, and that the glands are closely connected with sympathetic ganglia; while Kölliker also states that the posterior part of the organ is unpaired in the embryo rabbit of 16 or 17 days.
The structure and development of what I have called the interrenal bodyin Elasmobranchii so closely correspond with that of the mesoblastic part of the suprarenal bodies of the Reptilia, that I have very little hesitation in regarding them as homologous[241]; while the paired bodies in Elasmobranchii, derived from the sympathetic ganglia, clearly correspond with the part of the suprarenals of Reptilia having a similar origin; although the anterior parts of the paired suprarenal bodies of Fishes have clearly become aborted in the higher types.
In Elasmobranch Fishes we thus have (1) a series of paired bodies, derived from the sympathetic ganglia, and (2) an unpaired body of mesoblastic origin. In the Amniota these bodies unite to form the compound suprarenal bodies, the two constituents of which remain, however, distinct in their development. The mesoblastic constituent appears to form the cortical part of the adult suprarenal body, and the nervous constituent the medullary part.
Bibliographyof the Suprarenal bodies.
(503)M. Braun. “Bau u. Entwick. d. Nebennieren bei Reptilien.”Arbeit. a. d. zool.-zoot. Institut Würzburg,Vol.V. 1879.(504)A. v. Brunn. “Ein Beitrag z. Kenntniss d. feinern Baues u. d. Entwick. d. Nebennieren.”Archiv f. mikr. Anat.,Vol.VIII. 1872.(505)Fr. Leydig.Untersuch. üb. Fische u. Reptilien.Berlin, 1853.(506)Fr. Leydig.Rochen u. Haie.Leipzig, 1852.Videalso F. M. Balfour (No.292), Kölliker (No.298), Remak (No.302), etc.
[219]For a full account of these structures the reader is referred to T. W. Bridge, “Pori Abdominales of Vertebrata.”Journal of Anat. and Physiol.,Vol.XIV., 1879.[220]Kölliker’s account of this septum, which he calls the mesocardium laterale (No.298,p.295), would seem to imply that in Mammals it is completed posteriorly even before the formation of the liver. I doubt whether this takes place quite so early as he implies, but have not yet determined its exact period by my own observations.[221]“Connective and vasifactive tissues of the Leech.”Quart. J. of Micr. Science,Vol.XX. 1880.[222]VideGegenbaur,“Zur vergleich. Anat. d. Herzens.”Jenaische Zeit.,Vol.II. 1866, and for recent important observations, J. E. V. Boas,“Ueb. Herz u. Arterienbogen bei Ceratoden u. Protopterus,”and“Ueber d. Conus arter. b. Butirinus, etc.,”Morphol. Jahrb.,Vol.VI. 1880.[223]Boas holds that the longitudinal septum is formed by the coalescence of a row of longitudinal valves, but this is opposed to Lankester’s statements, “On the hearts of Ceratodus, Protopterus and Chimæra, etc.”Zool. Trans.Vol.X.1879.[224]For a good description of the adult heartvideHuxley, Article “Amphibia,” in theEncyclopædia Britannica.[225]It is just possible that the reverse may be true,videnote onp.640. If however, as is most probable, the statement in the text is correct, the valves at the mouth of the ventricle in Teleostei are not homologous with those of the Amniota; the former being the distal row of the valves of the conus, the latter the proximal.[226]If Tonge is correct in his statement that the semilunar valves develop at some distance from the mouth of the ventricle, it would seem possible that the portion of the truncus between them and the ventricle ought to be regarded as the embryonic conus arteriosus, and that the distal row of valves of the conus (and not the proximal as suggested above,p.639) has been preserved in the higher types.[227]In Mammalia the superior mesenteric artery arises from the vitelline artery, which may probably be regarded as a primitive cæliaco-mesenteric artery.[228]The mandibular artery is stated by Götte never to be developed in Teleostei, but is distinctly figured in Lereboullet (No.71).[229]In my account of the Amphibia, Götte (No.296) has been followed.[230]His (No.232) describes in Man two ventral continuations of the truncus arteriosus, one derived from the mandibular artery, forming the external maxillary artery, and one from the hyoid artery, forming the lingual artery. The vessel from which they spring is the external carotid. These observations of His will very probably be found to hold true for other types.[231]J. Müller holds that this sack is not rhythmically contractile.[232]Stannius,Vergleich. Anat.,p.251.[233]This statement of Götte’s is opposed to that of Rathke for the Amniota, and cannot be considered as completely established.[234]Rathke’s account of the vena renalis advehens is thus entirely opposed to that which Götte gives for the Frog, but my own observations on the Lizard incline me to accept Rathke’s statements, for the Amniota at any rate.[235]The vena cava inferior does not according to Rathke’s account unite behind with the posterior cardinal veins, as it is stated by Götte to do in the Anura. Götte questions the accuracy of Rathke’s statements on this head, but my own observations are entirely in favour of Rathke’s observations, and lend no support whatever to Götte’s views.[236]The junction between the portal system and the anterior abdominal vein is apparently denied by Rathke (No.300,p.173), but this must be an error on his part.[237]The mode in which this is effected requires further investigation.[238]Rathke, as mentioned above, holds that in the Snake the front part of the posterior cardinals completely aborts. Further investigations are required to shew whether there really is a difference between Mammalia and Reptilia in this matter.[239]According to Rathke the original trunk connecting the allantoic vein directly with the heart through the liver is aborted, and the ductus venosus Arantii is a secondary connection established in the latter part of fœtal life.[240]These globules are not formed of a true fatty substance, and this is also probably true for the similar globules of the interrenal bodies of Elasmobranchii.[241]The fact of the organ being unpaired in Elasmobranchii and paired in the Amniota is of no importance, as is shewn by the fact that part of the organ is unpaired in the Rabbit.
[219]For a full account of these structures the reader is referred to T. W. Bridge, “Pori Abdominales of Vertebrata.”Journal of Anat. and Physiol.,Vol.XIV., 1879.
[220]Kölliker’s account of this septum, which he calls the mesocardium laterale (No.298,p.295), would seem to imply that in Mammals it is completed posteriorly even before the formation of the liver. I doubt whether this takes place quite so early as he implies, but have not yet determined its exact period by my own observations.
[221]“Connective and vasifactive tissues of the Leech.”Quart. J. of Micr. Science,Vol.XX. 1880.
[222]VideGegenbaur,“Zur vergleich. Anat. d. Herzens.”Jenaische Zeit.,Vol.II. 1866, and for recent important observations, J. E. V. Boas,“Ueb. Herz u. Arterienbogen bei Ceratoden u. Protopterus,”and“Ueber d. Conus arter. b. Butirinus, etc.,”Morphol. Jahrb.,Vol.VI. 1880.
[223]Boas holds that the longitudinal septum is formed by the coalescence of a row of longitudinal valves, but this is opposed to Lankester’s statements, “On the hearts of Ceratodus, Protopterus and Chimæra, etc.”Zool. Trans.Vol.X.1879.
[224]For a good description of the adult heartvideHuxley, Article “Amphibia,” in theEncyclopædia Britannica.
[225]It is just possible that the reverse may be true,videnote onp.640. If however, as is most probable, the statement in the text is correct, the valves at the mouth of the ventricle in Teleostei are not homologous with those of the Amniota; the former being the distal row of the valves of the conus, the latter the proximal.
[226]If Tonge is correct in his statement that the semilunar valves develop at some distance from the mouth of the ventricle, it would seem possible that the portion of the truncus between them and the ventricle ought to be regarded as the embryonic conus arteriosus, and that the distal row of valves of the conus (and not the proximal as suggested above,p.639) has been preserved in the higher types.
[227]In Mammalia the superior mesenteric artery arises from the vitelline artery, which may probably be regarded as a primitive cæliaco-mesenteric artery.
[228]The mandibular artery is stated by Götte never to be developed in Teleostei, but is distinctly figured in Lereboullet (No.71).
[229]In my account of the Amphibia, Götte (No.296) has been followed.
[230]His (No.232) describes in Man two ventral continuations of the truncus arteriosus, one derived from the mandibular artery, forming the external maxillary artery, and one from the hyoid artery, forming the lingual artery. The vessel from which they spring is the external carotid. These observations of His will very probably be found to hold true for other types.
[231]J. Müller holds that this sack is not rhythmically contractile.
[232]Stannius,Vergleich. Anat.,p.251.
[233]This statement of Götte’s is opposed to that of Rathke for the Amniota, and cannot be considered as completely established.
[234]Rathke’s account of the vena renalis advehens is thus entirely opposed to that which Götte gives for the Frog, but my own observations on the Lizard incline me to accept Rathke’s statements, for the Amniota at any rate.
[235]The vena cava inferior does not according to Rathke’s account unite behind with the posterior cardinal veins, as it is stated by Götte to do in the Anura. Götte questions the accuracy of Rathke’s statements on this head, but my own observations are entirely in favour of Rathke’s observations, and lend no support whatever to Götte’s views.
[236]The junction between the portal system and the anterior abdominal vein is apparently denied by Rathke (No.300,p.173), but this must be an error on his part.
[237]The mode in which this is effected requires further investigation.
[238]Rathke, as mentioned above, holds that in the Snake the front part of the posterior cardinals completely aborts. Further investigations are required to shew whether there really is a difference between Mammalia and Reptilia in this matter.
[239]According to Rathke the original trunk connecting the allantoic vein directly with the heart through the liver is aborted, and the ductus venosus Arantii is a secondary connection established in the latter part of fœtal life.
[240]These globules are not formed of a true fatty substance, and this is also probably true for the similar globules of the interrenal bodies of Elasmobranchii.
[241]The fact of the organ being unpaired in Elasmobranchii and paired in the Amniota is of no importance, as is shewn by the fact that part of the organ is unpaired in the Rabbit.
In all the Cœlenterata, except the Ctenophora, the contractile elements of the body wall consist of filiform processes of ectodermal or entodermal epithelial cells (figs.375and376B). The elements provided with these processes, which were first discovered by Kleinenberg, are known asmyoepithelial cells. Their contractile parts may either be striated (fig. 376) or non-striated (fig. 375). In some instances the epithelial part of the cell may nearly abort, its nucleus alone remaining (fig. 376A); and in this way a layer of muscles lying completely below the surface may be established.
Illustration: Figure 375Fig. 375. Myo-epithelial cells of Hydra.(From Gegenbaur; after Kleinenberg.)m.contractile fibres.
Fig. 375. Myo-epithelial cells of Hydra.(From Gegenbaur; after Kleinenberg.)m.contractile fibres.
There is embryological evidence of the derivation of the voluntary muscular system of a large number of types from myoepithelial cells of this kind. The more important of these groups are the Chætopoda, the Gephyrea, the Chætognatha, the Nematoda, and the Vertebrata[242].
While there is clear evidence that the muscular system of a large number of types is composed of cells which had their origin in myoepithelial cells, the mode of evolution of themuscular system of other types is still very obscure. The muscles may arise in the embryo from amœboid or indifferent cells, and the Hertwigs[243]hold that in many of these instances the muscles have also phylogenetically taken their origin from indifferent connective-tissue cells. The subject is however beset with very serious difficulties, and to discuss it here would carry me too far into the region of pure histology.
The voluntary muscular system of the Chordata.
The muscular fibres. The muscular elements of the Chordata undoubtedly belong to the myoepithelial type. The embryonic muscle-cells are at first simple epithelial cells, but soon become spindle-shaped: part of their protoplasm becomes differentiated into longitudinally placed striated muscular fibrils, while part, enclosing the nucleus, remains indifferent, and constitutes the epithelial element of the cells. The muscular fibrils are either placed at one side of the epithelial part of the cell, or in other instances (the Lamprey, the Newt, the Sturgeon, the Rabbit) surround it. The latter arrangement is shewn for the Sturgeon infig. 57.
Illustration: Figure 376Fig. 376. Muscle-cells of Lizzia Köllikeri.(From Lankester; after O. and R. Hertwig.)A. Muscle-cell from the circular fibres of the subumbrella.B. Myoepithelial cells from the base of a tentacle.
Fig. 376. Muscle-cells of Lizzia Köllikeri.(From Lankester; after O. and R. Hertwig.)A. Muscle-cell from the circular fibres of the subumbrella.B. Myoepithelial cells from the base of a tentacle.
The number of the fibrils of each cell gradually increases, and the protoplasm diminishes, so that eventually only the nucleus, or nuclei resulting from its division, are left. The products of each cell probably give rise, in conjunction with a further division of the nucleus, to a primitive bundle, which,except in Amphioxus, Petromyzon, etc., is surrounded by a special investment of sarcolemma.
The voluntary muscular system. For the purposes of description the muscular system of the Vertebrata may conveniently be divided into two sections,viz.that of the head and that of the trunk. The main part, if not the whole, of the muscular system of the trunk is derived from certain structures, known as the muscle-plates, which take their origin from part of the primitive mesoblastic somites.
Illustration: Figure 377Fig. 377. Transverse section through the trunk of an embryo slightly older than fig. 28 E.nc.neural canal;pr.posterior root of spinal nerve;x.subnotochordal rod;ao.aorta;sc.somatic mesoblast;sp.splanchnic mesoblast;mp.muscle-plate;mp´.portion of muscle-plate converted into muscle;Vr.portion of the vertebral plate which will give rise to the vertebral bodies;al.alimentary tract.
Fig. 377. Transverse section through the trunk of an embryo slightly older than fig. 28 E.nc.neural canal;pr.posterior root of spinal nerve;x.subnotochordal rod;ao.aorta;sc.somatic mesoblast;sp.splanchnic mesoblast;mp.muscle-plate;mp´.portion of muscle-plate converted into muscle;Vr.portion of the vertebral plate which will give rise to the vertebral bodies;al.alimentary tract.
It has already been stated (pp. 292-296) that the mesoblastic somites are derived from the dorsal segmented part of the primitive mesoblastic plates. Since the history of these bodies is presented in its simplest form in Elasmobranchii it will be convenient to commence with this group. Each somite is composed of two layers—a somatic and a splanchnic—both formed of a single row of columnar cells. Between these two layers is a cavity, which is at first directly continuous with the general body cavity, of which indeed it merely forms a specialised part (fig. 377). Before long the cavity becomes however completely constricted off from the permanent body cavity.
Very early (fig. 377) the inner or splanchnic wall of the somites loses its simple constitution, owing to the middle part of it undergoing peculiar changes. The meaning of the changes is at once shewn by longitudinal horizontal sections, which prove (fig. 378) that the cells in this situation (mp´) have become extended in a longitudinal direction, and, in fact, form typical spindle-shaped embryonic muscle-cells, each with a large nucleus. Every muscle-cell extends for the whole length of a somite. The inner layer of each somite, immediately within the muscle-band just described, begins to proliferate, and producea mass of cells, placed between the muscles and the notochord (Vr). These cells form the commencing vertebral bodies, and have at first (fig. 378) the same segmentation as the somites from which they sprang.
After the separation of the vertebral bodies from the somites the remaining parts of the somites may be called muscle-plates; since they become directly converted into the whole voluntary muscular system of the trunk (fig. 379,mp).
According to the statements of Bambeke and Götte, the Amphibians present some noticeable peculiarities in the development of their muscular system, in that such distinct muscle-plates as those of other vertebrate types are not developed. Each side-plate of mesoblast is divided into a somatic and a splanchnic layer, continuous throughout the vertebral and parietal portions of the plate. The vertebral portions (somites) of the plates soon become separated from the parietal, and form independent masses of cells constituted of two layers, which were originally continuous with the somatic and splanchnic layers of the parietal plates (fig. 79). The outer or somatic layer of the vertebral plates is formed of a single row of cells, but the inner or splanchnic layer is made up of a kernel of cells on the side of the somatic layer and an inner layer. The kernel of the splanchnic layer and the outer or somatic layer together correspond to a muscle-plate of other Vertebrata, and exhibit a similar segmentation.
Osseous Fishes are stated to agree with Amphibians in the development of their somites and muscular systems[244], but further observations on this point are required.
Illustration: Figure 378Fig. 378. Horizontal section through the trunk of an embryo of Scyllium considerably younger than 28 F.ch.notochord;ep.epiblast;Vr.rudiment of vertebral body;mp.muscle-plate;mp´.portion of muscle-plate already differentiated into longitudinal muscles.
Fig. 378. Horizontal section through the trunk of an embryo of Scyllium considerably younger than 28 F.ch.notochord;ep.epiblast;Vr.rudiment of vertebral body;mp.muscle-plate;mp´.portion of muscle-plate already differentiated into longitudinal muscles.
In Birds the horizontal splitting of the mesoblast extends at first to the dorsal summit of the mesoblastic plates, but after the isolation of the somites the split between the somatic and splanchnic layers becomes to a large extent obliterated, though in the anterior somites it appears in part to persist. The somites on the second day, as seen in a transverse section (fig. 115,P.v.), are somewhat quadrilateral in form but broader than they are deep.
Each at that time consists of a somewhat thick cortex of radiatingrather granular columnar cells, enclosing a small kernel of spherical cells. They are not, as may be seen in the above figure, completely separated from the ventral (or lateral as they are at this period) parts of the mesoblastic plate, and the dorsal and outer layer of the cortex of the somites is continuous with the somatic layer of mesoblast, the remainder of the cortex, with the central kernel, being continuous with the splanchnic layer. Towards the end of the second and beginning of the third day the upper and outer layer of the cortex, together probably with some of the central cells of the kernel, becomes separated off as a muscle-plate (fig. 116). The muscle-plate when formed (fig. 117) is found to consist of two layers, an inner and an outer, which enclose between them an almost obliterated central cavity; and no sooner is the muscle-plate formed than the middle portion of the inner layer becomes converted into longitudinal muscles. The avian muscle-plates have, in fact, precisely the same constitution as those of Elasmobranchii. The central space is clearly a remnant of thevertebral portion of the body cavity, which, though it wholly or partially disappears in a previous stage, reappears again on the formation of the muscle-plate.
The remainder of the somite, after the formation of the muscle-plate, is of very considerable bulk; the cells of the cortex belonging to it lose their distinctive characters, and the major part of it becomes the vertebral rudiment.
In Mammalia the history appears to be generally the same as in Elasmobranchii. The split which gives rise to the body cavity is continued to the dorsal summit of the mesoblastic plates, and the dorsal portions of the plates with their contained cavities become divided into somites, and are then separated off from the ventral. The later development of the somites has not been worked out with the requisite care, but it would seem that they form somewhat cubical bodies in which all trace of the primitive slit is lost. The further development resembles that in Birds.
The first changes of the mesoblastic somites and the formation of the muscle-plates do not, according to existing statements, take place on quite the same type throughout the Vertebrata, yet the comparison which has been instituted between Elasmobranchs and other Vertebrates appears to prove that there are important common features in their development, which may be regarded as primitive, and as having been inherited from the ancestors of Vertebrates. These features are (1) the extension of the body cavity into the vertebral plates, and subsequent enclosure of this cavity between the two layers of the muscle-plates; (2) the primitive division of the vertebral plate into an outer (somatic) and an inner (splanchnic) layer, and the formation of a large part of the voluntary muscular system out of the innerlayer, which in all cases is converted into muscles earlier than the outer layer.
The conversion of the muscle-plates into muscles. It will be convenient to commence this subject with a description of the changes which take place in such a simple type as that of the Elasmobranchii.
Illustration: Figure 379Fig. 379. Section through the trunk of a Scyllium embryo slightly younger than 28 F.sp.c.spinal canal;W.white matter of spinal cord;pr.posterior nerve-roots;ch.notochord;x.subnotochordal rod;ao.aorta;mp.muscle-plate;mp´.inner layer of muscle-plate already converted into muscles;Vr.rudiment of vertebral body;st.segmental tube;sd.segmental duct;sp.v.spiral valve;v.subintestinal vein;p.o.primitive generative cells.
Fig. 379. Section through the trunk of a Scyllium embryo slightly younger than 28 F.sp.c.spinal canal;W.white matter of spinal cord;pr.posterior nerve-roots;ch.notochord;x.subnotochordal rod;ao.aorta;mp.muscle-plate;mp´.inner layer of muscle-plate already converted into muscles;Vr.rudiment of vertebral body;st.segmental tube;sd.segmental duct;sp.v.spiral valve;v.subintestinal vein;p.o.primitive generative cells.
At the time when the muscle-plates have become independent structures they form flat two-layered oblong bodies enclosing a slit-like central cavity (fig. 379,mp). The outer or somatic wall is formed of simple epithelial-like cells. The inner or splanchnic wall has however a somewhat complicated structure. It is composed dorsally and ventrally of a columnar epithelium, but in its middle portion of the muscle-cells previously spoken of. Between these and the central cavity of the plates the epithelium forming the remainder of the layer commences to insert itself; so that between the first-formed muscle and the cavity of the muscle-plate there appears a thin layer of cells, not however continuous throughout.
When first formed the muscle-plates, as viewed from the exterior, have nearly straight edges; soon however they become bent in the middle, so that the edges have an obtusely angular form, the apex of the angle being directed forwards. They are so arranged that the anterior edge of the one plate fits into the posterior edge of the one in front. In the lines of junction between the plates layers of connective-tissue cells appear, which form the commencements of the intermuscular septa.
The growth of the plates is very rapid, and their upper endssoon extend to the summit of the neural canal, and their lower ones nearly meet in the median ventral line. The original band of muscles, whose growth at first is very slow, now increases with great rapidity, and forms the nucleus of the whole voluntary muscular system (fig. 380,mp´). It extends upwards and downwards by the continuous conversion of fresh cells of the splanchnic layer into muscle-cells. At the same time it grows rapidly in thickness by the addition of fresh spindle-shaped muscle-cells from thesomatic layeras well as by the division of the already existing cells.
Thus both layers of the muscle plate are concerned in forming the great longitudinal lateral muscles, though the splanchnic layer is converted into muscles very much sooner than the somatic[245].
Each muscle-plate is at first a continuous structure, extending from the dorsal to the ventral surface, but after a time it becomes divided by a layer of connective tissue, which becomes developed nearly on a level with the lateral line, into adorso-lateraland aventro-lateralsection. The ends of the muscle-plates continue for a long time to be formed of undifferentiated columnar cells. The complicated outlines of the intermuscular septa become gradually established during the later stages of development, causing the well-known appearances of the muscles in transverse sections, which require no special notice here.
Illustration: Figure 380Fig. 380. Transverse section through the anterior part of the trunk of an embryo of Scyllium slightly older than fig. 29 B.The section is diagrammatic in so far that the anterior nerve-roots have been inserted for the whole length; whereas they join the spinal cord halfway between two posterior roots.sp.c.spinal cord;sp.g.ganglion of posterior root;ar.anterior root;dn.dorsally directed nerve springing from posterior root;mp.muscle-plate;mp´.part of muscle-plate already converted into muscles;m.pl.part of muscle-plate which gives rise to the muscles of the limbs;nl.nervus lateralis;ao.aorta;ch.notochord;sy.g.sympathetic ganglion;ca.v.cardinal vein;sp.n.spinal nerve;sd.segmental (archinephric) duct;st.segmental tube;du.duodenum;pan.pancreas;hp.d.point of junction of hepatic duct with duodenum;umc.umbilical canal.
Fig. 380. Transverse section through the anterior part of the trunk of an embryo of Scyllium slightly older than fig. 29 B.The section is diagrammatic in so far that the anterior nerve-roots have been inserted for the whole length; whereas they join the spinal cord halfway between two posterior roots.sp.c.spinal cord;sp.g.ganglion of posterior root;ar.anterior root;dn.dorsally directed nerve springing from posterior root;mp.muscle-plate;mp´.part of muscle-plate already converted into muscles;m.pl.part of muscle-plate which gives rise to the muscles of the limbs;nl.nervus lateralis;ao.aorta;ch.notochord;sy.g.sympathetic ganglion;ca.v.cardinal vein;sp.n.spinal nerve;sd.segmental (archinephric) duct;st.segmental tube;du.duodenum;pan.pancreas;hp.d.point of junction of hepatic duct with duodenum;umc.umbilical canal.
The muscles of the limbs. The limb muscles are formed in Elasmobranchii, coincidently with the cartilaginous skeleton, as two bands of longitudinal fibres on the dorsal and ventral surfaces of the limbs (fig. 346). The cells, from which these muscles originate, are derived from the muscle-plates. When the ends of the muscle-plates reach the level of the limbs they bend outwards and enter the tissue of the limbs (fig. 380). Small portions of several muscle-plates (m.pl) come in this way to be situated within the limbs, and are very soon segmented off from the remainder of the muscle-plates. The portions of the muscle-plates thus introduced soon lose their original distinctness.There can however be but little doubt that they supply the tissue for the muscles of the limbs. The muscle-plates themselves, after giving off buds to the limbs, grow downwards, and soon cease to shew any trace of having given off these buds.
In addition to the longitudinal muscles of the trunk just described, which are generally characteristic of Fishes, there is found in Amphioxus a peculiar transverse abdominal muscle, extending from the mouth to the abdominal pore, the origin of which has not been made out.
It has already been shewn that in all the higher Vertebrata muscle-plates appear, which closely resemble those in Elasmobranchii; so that all the higher Vertebrata pass through, with reference to their muscular system, a fish-like stage. The middle portion of the inner layers of their muscle-plates becomes, as in Elasmobranchii, converted into muscles at a very early period, and the outer layer for a long time remains formed of indifferent cells. That these muscle-plates give rise to the main muscular system of the trunk, at any rate to the episkeletal muscles of Huxley, is practically certain, but the details of the process have not been made out.
In the Perennibranchiata the fish-like arrangement of muscles is retained through life in the tail and in the dorso-lateral parts of the trunk. In the tail of the Amniotic Vertebrata the primitive arrangement is also more or less retained, and the same holds good for the dorso-lateral trunk muscles of the Lacertilia. In the other Amniota and the Anura the dorso-lateral muscles have become divided up into a series of separate muscles, which are arranged in two main layers. It is probable that the intercostal muscles belong to the same group as the dorso-lateral muscles.
The abdominal muscles of the trunk, even in the lowest Amphibia, exhibit a division into several layers. The recti abdominis are the least altered part of this system, and usually retain indications of the primitive intermuscular septa, which in many Amphibia and Lacertilia are also to some extent preserved in the other abdominal muscles.
In the Amniotic Vertebrates there is formed underneath the vertebral column and the transverse processes a system of muscles, forming part of the hyposkeletal system of Huxley, and called by Gegenbaur the subvertebral muscles. The development of this system has not been worked out, but on the whole I am inclined to believe that it is derived from the muscle-plates. Kölliker, Huxley and other embryologists believe however that these muscles are independent of the muscle-plates in their origin.
Whether the muscle of the diaphragm is to be placed in the same category as the hyposkeletal muscles has not been made out.
It is probable that the cutaneous muscles of the trunk are derived from the cells given off from the muscle-plates. Kölliker however believes that they have an independent origin.
The limb-muscles, both extrinsic and intrinsic, as may be concluded from their development in Elasmobranchii, are derived from the muscle-plates. Kleinenberg found in Lacertilia a growth of the muscle-plates into the limbs, and in Amphibia Götte finds that the outer layer of the muscle-plates gives rise to the muscles of the limbs.
In the higher Vertebrata on the other hand the entrance of the muscle-plates into the limbs has not been made out (Kölliker). It seems therefore probable that by an embryological modification, of which instances are so frequent, the cells which give rise to the muscles of the limbs in the higher Vertebrata can no longer be traced into a direct connection with the muscle-plates.
The Somites and muscular system of the head.
The extension of the somites to the anterior end of the body in Amphioxus clearly proves that somites, similar to those of the trunk, were originally present in a region, which in the higher Vertebrata has become differentiated into the head. In the adult condition no true Vertebrate exhibits indications of such somites, but in the embryos of several of the lower Vertebrata structures have been found, which are probably equivalent to the somites of the trunk: they have been frequently alluded to in the previous chapters of this volume. These structures have been most fully worked out in Elasmobranchii.
The mesoblast in Elasmobranch embryos becomes first split into somatic and splanchnic layers in the region of the head; and between these layers there are formed two cavities, one on each side, which end in front opposite the blind anterior extremity of the alimentary canal; and are continuous behind with the general body-cavity (fig. 20A,vp). I propose calling them thehead-cavities. The cavities of the two sides have no communication with each other.
Coincidently with the formation of an outgrowth from the throat to form the first visceral cleft, the head-cavity on each side becomes divided into a section in front of the cleft and a section behind the cleft; and at a later period it becomes, owing to the formation of a second cleft, divided into three sections:(1) a section in front of the first or hyomandibular cleft; (2) a section in the hyoid arch between the hyomandibular cleft and the hyobranchial or first branchial cleft; (3) a section behind the first branchial cleft.