[194]The function of the otoliths is not always clear. There is evidence to shew that they sometimes act as dampers.[195]For the somewhat complicated details as to the development of the auditory sacks of Cephalopods I must refer the reader toVol.II.,pp.278, 279, and to Grenacher (Vol.I., No.280).[196]It is not clear from Böttcher’s description how it comes about that the inner rods of Corti are more numerous than the outer.[197]The organs which resemble those of the lateral line are the remarkable sense organs found by Eisig in the Capitellidæ (Mittheil. a. d. Zool. Station zu Neapel,Vol.1); but I am not inclined to think that there is a true homology between these organs and the lateral line of Vertebrata. It seems to me probable that the segmentally arranged optic organs of Polyophthalmus are a special modification of the more indifferent sense organs of the Capitellidæ. The close affinity of these two types of Chætopods is favourable to this view.[198]Götte and Semper both hold that the lateral nerve, instead of growing in a centrifugal manner like other nerves, is directly derived from the epiblast of the lateral line. For the reasons which prevent me accepting this view I must refer the reader to myMonograph on Elasmobranch Fishes,pp.141-146.
[194]The function of the otoliths is not always clear. There is evidence to shew that they sometimes act as dampers.
[195]For the somewhat complicated details as to the development of the auditory sacks of Cephalopods I must refer the reader toVol.II.,pp.278, 279, and to Grenacher (Vol.I., No.280).
[196]It is not clear from Böttcher’s description how it comes about that the inner rods of Corti are more numerous than the outer.
[197]The organs which resemble those of the lateral line are the remarkable sense organs found by Eisig in the Capitellidæ (Mittheil. a. d. Zool. Station zu Neapel,Vol.1); but I am not inclined to think that there is a true homology between these organs and the lateral line of Vertebrata. It seems to me probable that the segmentally arranged optic organs of Polyophthalmus are a special modification of the more indifferent sense organs of the Capitellidæ. The close affinity of these two types of Chætopods is favourable to this view.
[198]Götte and Semper both hold that the lateral nerve, instead of growing in a centrifugal manner like other nerves, is directly derived from the epiblast of the lateral line. For the reasons which prevent me accepting this view I must refer the reader to myMonograph on Elasmobranch Fishes,pp.141-146.
Introduction.
Amongst the products of that part of the mesoblast which constitutes the connective tissue of the body special prominence must be given to the skeleton of the Vertebrata, from its importance in relation to numerous phylogenetic and morphological problems.
The development of the skeleton is however so large a subject that it cannot be satisfactorily dealt with except in a special treatise devoted to it; and the following description must be regarded as a mere sketch, from which detail has been as far as possible excluded.
In the lowest Chordata the sole structure present, which deserves to be called a skeleton, is the notochord. Although the notochord often persists as an important organ in the true Vertebrata, yet there are always added to it various skeletal structures developed in the mesoblast. Before entering into a systematic description of these, it will be convenient to say a few words as to the general characters of the skeleton.
Two elements, distinct both in their genesis and structure, are to be recognized in the skeleton. The one, forming the true primitive internal skeleton or endoskeleton, is imbedded within the muscles and is originally formed in cartilage. In many instances it retains a cartilaginous consistency through life, but in the majority of cases it becomes gradually ossified, andconverted into true bone. Bones so formed are known as cartilage bones.
The other element is originally formed by the fusion of the ossified bases of the dermal placoid scales already described in ChapterXIV., or by the fusion of the ossified bases of teeth situated in the mucous membrane of the mouth. In both instances the plates of bone so formed may lose the teeth or spines with which they were in the first instance covered, either by absorption in the individual, or phylogenetically by their gradually ceasing to be developed. The plates of bone, which originated by the above process, become in higher types directly developed in the connective tissue beneath the skin; and gradually acquire a deeper situation, and are finally so intimately interlocked with parts of the true internal skeleton, that the two sets of elements can only be distinguished by the fact of the one set ossifying in cartilage and the other in membrane.
It seems probable that in the Reptilia, and possibly the extinct Amphibia, dermal bones have originated in the skin without the intervention of superjacent spinous structures.
In cases where a membrane bone, as the dermal ossifications are usually called, overlies a part of the cartilage, it may set up ossification in the latter, and the cartilage bone and membrane bone may become so intimately fused as to be quite inseparable. It seems probable that in cases of this kind the compound bone may in the course of further evolution entirely lose either its cartilaginous element or its membranous element; so that cases occasionally occur where the development of a bone ceases to be an absolutely safe guide to its evolution.
As to the processes which take place in the ossification of cartilage there is still much to be made out. Two processes are often distinguished,viz.(1) a process known as ectostosis, in which the ossification takes place in the perichondrium, and either simply surrounds or gradually replaces the cartilage, and (2) a process known as endostosis, where the ossification actually takes place between the cartilage cells. It seems probable however (Gegenbaur, Vrolik) that there is no sharp line to be drawn between these two processes; but that the ossification almost always starts from the perichondrium. In the higher types, as a rule, the vessels of the perichondrium extend intothe cartilage, and the ossification takes place around these vessels within the cartilage; but in the lower types (Pisces, Amphibia) ossification is often entirely confined to the perichondrium; and the cartilage is simply absorbed.
The regions where ossification first sets in are known as centres of ossification; and from these centres the ossification spreads outwards. There may be one or more centres for a bone.
The actual causes which in the first instance gave rise to particular centres of ossification, or to the ossification of particular parts of the cartilage, are but little understood; nor have we as yet any satisfactory criterion for determining the value to be attached to the number and position of centres of ossification. In some instances such centres appear to have an important morphological significance, and in other instances they would seem to be determined by the size of the cartilage about to be ossified.
There is no doubt that the membrane bones and cartilage bones can as a rule be easily distinguished by their mode of development; but it is by no means certain that this is always the case. It is necessarily very difficult to establish the homology between bones, which develop in one type from membrane and in another type from cartilage; but there are without doubt certain instances in which the homology between two bones would be unhesitatingly admitted were it not for the difference in their development. The most difficult cases of this kind are connected with the shoulder-girdle.
The possible sources of confusion in the development of bones are obviously two. (1) A cartilage bone by origin may directly ossify in membrane, without the previous development of cartilage, and (2) a membrane bone may in the first instance be formed in cartilage.
The occurrence of the first of these is much more easy to admit than that of the second; and there can be little doubt that it sometimes takes place. In a large number of cases it would moreover cause no serious difficulty to the morphologist.
Bibliographyof the origin of the Skeleton.
(405)C. Gegenbaur. “Ueb. primäre u. secundäre Knochenbildung mit besonderer Beziehung auf d. Lehre von dem Primordialcranium.”Jenaische Zeitschrift,Vol.III. 1867.(406)O. Hertwig. “Ueber Bau u. Entwicklung d. Placoidschuppen u. d. Zähne d. Selachier.”Jenaische Zeitschrift,Vol.VIII. 1874.(407)O. Hertwig. “Ueb. d. Zahnsystem d. Amphibien u. seine Bedeutung f. d. Genese d. Skelets d. Mundhöhle.”Archiv f. mikr. Anat.,Vol.XI. Supplementheft, 1874.(408)O. Hertwig. “Ueber d. Hautskelet d. Fische.”Morphol. Jahrbuch,Vol.II. 1876. (Siluroiden u. Acipenseriden.)(409)O. Hertwig. “Ueber d. Hautskelet d. Fische (Lepidosteus u. Polypterus).”Morph. Jahrbuch,Vol.V. 1879.(410)A. Kölliker. “Allgemeine Betrachtungen üb. die Entstehung d. knöchernen Schädels d. Wirbelthiere.”Berichte v. d. königl. zoot. Anstalt z. Würzburg, 1849.(411)Fr. Leydig. “Histologische Bemerkungen üb. d. Polypterus bichir.”Zeit. f. wiss. Zool.,Vol.V. 1858.(412)H. Müller. “Ueber d. Entwick. d. Knochensubstanz nebst Bemerkungen, etc.”Zeit. f. wiss. Zool.,Vol.IX. 1859.(413)Williamson. “On the structure and development of the Scales and Bones of Fishes.”Phil. Trans., 1851.(414)Vrolik. “Studien üb. d. Verknöcherung u. die Knochen d. Schädels d. Teleostier.”Niederländisches Archiv f. Zoologie,Vol.I.
Notochord and Vertebral column.
The primitive axial skeleton of the Chordata consists of the notochord and its sheath. It persists as such in the adult in Amphioxus, and constitutes, in embryos of all Vertebrata, for a considerable period of their early embryonic life, the sole representative of the axial skeleton.
Illustration: Figure 313Fig. 313. Horizontal section through the trunk of an embryo of Scyllium considerably younger than F in fig. 28.The section is taken at the level of the notochord, and shews the separation of the cells to form the vertebral bodies from the muscle-plates.ch.notochord;ep.epiblast;Vr.rudiment of vertebral body;mp.muscle-plate;mp´.portion of muscle-plate already differentiated into longitudinal muscles.
Fig. 313. Horizontal section through the trunk of an embryo of Scyllium considerably younger than F in fig. 28.The section is taken at the level of the notochord, and shews the separation of the cells to form the vertebral bodies from the muscle-plates.ch.notochord;ep.epiblast;Vr.rudiment of vertebral body;mp.muscle-plate;mp´.portion of muscle-plate already differentiated into longitudinal muscles.
The Notochord. The early formation of the notochord has already been described in detail (pp.292-300). It is developed, in most if not all cases, as an axial differentiation of the hypoblast, and forms at first a solid cord of cells, without a sheath, placed between the nervous system and the dorsal wall of the alimentary tract, and extending from the base of the front of the mid-brain to the end of the tail. The section in the region of the brain will be dealt with by itself. Thatin the trunk forms the basis round which the vertebral column is moulded.
The early histological changes in the cells of the notochord are approximately the same in all the Craniata. There is formed by the superficial cells of the notochord a delicate sheath, which soon thickens, and becomes a well-defined structure. Vacuoles (one or more to each cell) are formed in the cells of the notochord, which enlarge till the whole notochord becomes almost entirely formed of large vacuoles separated by membranous septa which form a complete sponge-like reticulum (fig. 313). In the Ichthyopsida most of the protoplasm with the nuclei is carried to the periphery, where it forms a special nucleated layer sometimes divided into definite epithelial-like cells (fig. 314), while in the meshes of the reticulum a few nuclei surrounded by a little protoplasm still remain. In the Amniotic Vertebrata, probably owing to the early atrophy of the notochord, the distribution of the nuclei in the spaces of the mesh-work remains fairly uniform.
Illustration: Figure 314Fig. 314. Section through the spinal column of a young Salmon.(From Gegenbaur.)cs.sheath of notochord;k.neural arch;k´.hæmal arch;m.spinal cord;a.dorsal aorta;v.cardinal veins.
Fig. 314. Section through the spinal column of a young Salmon.(From Gegenbaur.)cs.sheath of notochord;k.neural arch;k´.hæmal arch;m.spinal cord;a.dorsal aorta;v.cardinal veins.
In the early stages of development the spaces in the notochordal sponge-work, each containing a nucleus and protoplasm, probably represent cells. In the types in which the notochord persists in the adult the mesh-work becomes highly complicated, and then forms a peculiar reticulum filled with gelatinous material, the spaces in which do not indicate the outlines of definite cells (figs.315and318).
Around the sheath of the notochord there is formed in the Cyclostomata, Ganoidei, Elasmobranchii and Teleostei an elastic membrane usually known as the membrana elastica externa.
In most Vertebrates the notochord and its sheath either atrophy completely or become a relatively unimportant part of the axial skeleton; but in the Cyclostomata (fig. 315) and in the Selachioidean Ganoids (Acipenser, etc.) they persist as thesole representative of the true vertebral axis. The sheath becomes very much thickened; and on the membrana elastica coveringit the vertebral arches directly rest. In Elasmobranchii the sheath of the notochord undergoes a more complicated series of changes, which result first of all in the formation of a definiteunsegmentedcartilaginous tube[199]round the notochord, and subsequently (in most forms) in the formation of true vertebral bodies.
Illustration: Figure 315Fig. 315. Section through the vertebral column of Ammocœtes.(From Gegenbaur.)Ch.notochord;cs.notochordal sheath;m.spinal cord;a.aorta;v.cardinal veins.
Fig. 315. Section through the vertebral column of Ammocœtes.(From Gegenbaur.)Ch.notochord;cs.notochordal sheath;m.spinal cord;a.aorta;v.cardinal veins.
Between the membrana elastica externa and the sheath of the notochord a layer of cells becomes interposed (fig. 316,n), which lie in a matrix not sharply separated from the sheath of the notochord. The cells which form this layer appear to be derived from a special investment of the notochord, and to have penetrated through the membrana elastica externa to reach their final situation. The layer with these cells soon increases in thickness, and forms a continuous unsegmented tube of fibrous tissue with flattened concentrically arranged nuclei (fig. 317,Vb). Externally is placed the membrana elastica externa (mel), while within is the cuticular sheath of the notochord. This tube is the cartilaginous tube spoken of above and is known as the cartilaginous sheath of the notochord.
Illustration: Figure 316Fig. 316. Longitudinal section through a small part of the notochord and adjoining parts of a Scyllium embryo, at the time of the first formation of the cartilaginous sheath.ch.notochord;sc.sheath of notochord;n.nuclei of cartilaginous sheath;me.e.membrana elastica externa.
Fig. 316. Longitudinal section through a small part of the notochord and adjoining parts of a Scyllium embryo, at the time of the first formation of the cartilaginous sheath.ch.notochord;sc.sheath of notochord;n.nuclei of cartilaginous sheath;me.e.membrana elastica externa.
The exact origin of the cartilaginous tube just described is a question of fundamental importance with reference to the origin of the vertebral column and the homologies of its constituent parts; but is by no means easy to settle. In the account of the subject in my memoir onElasmobranch FishesI held with Gegenbaur that it arose froma layer of cellsoutsidethe sheath of the notochord, on the exterior of which the membrana elastica externa was subsequently formed. To this view Götte (No.419) also gave his adhesion. Schneider has since (No.429) stated that this is not the case, but that, as described above, the membrana elastica externa is formed before the layer of cartilage. I have since worked over this subject again, and am on the whole inclined to adopt Schneider’s correction.
It follows from the above description that the cartilaginous tube in question is an essential part of the sheath of the notochord, and that it is to some extent homologous with the notochordal sheath of the Sturgeon and the Lamprey, and not an entirely new formation.
Illustration: Figure 317Fig. 317. Transverse section through the ventral part of the notochord and adjoining structures of an advanced Scyllium embryo at the root of the tail.Vb.cartilaginous sheath of the notochord;ha.hæmal arch;vp.process to which the rib is articulated;mel.membrana elastica externa;ch.notochord;ao.aorta;V.cau.caudal vein.
Fig. 317. Transverse section through the ventral part of the notochord and adjoining structures of an advanced Scyllium embryo at the root of the tail.Vb.cartilaginous sheath of the notochord;ha.hæmal arch;vp.process to which the rib is articulated;mel.membrana elastica externa;ch.notochord;ao.aorta;V.cau.caudal vein.
This sheath forms the basis of the centra of the future vertebræ. In a few adult forms,i.e.Chimæra and the Dipnoi, it retains its primitive condition, except that in Chimæra there are present delicate ossified rings more numerous than the arches; while in the Notidani, Læmargi and Echinorhini theindications of vertebræ are imperfectly marked out. The further history of this sheath in the forms in which true vertebræ are formed can only be dealt with in connection with the formation of the vertebral arches.
In Teleostei there is present, as in Elasmobranchii, an elastica externa, and an inner notochordal sheath. The elastica externa contains, according to Götte, cells. These cells, if present, are however very difficult to make out, but in any case the so-called elastica externa appears to correspond with the cartilaginous sheath of Elasmobranchii together with its enveloping elastica, since ossification, when it sets in, occurs in this layer. The sheath within becomes unusually thick.
In the Amphibia and in the Amniota no membrane is present which can be identified with the membrana elastica externa of the Elasmobranchii, Teleostei, etc. In Amphibia (Götte) there is formed round the notochord a cellular sheath, which has very much the relations of the cartilaginous tube around the notochord of Elasmobranchii, and is developed in the same way from the perichordal connective tissue cells. It is only necessary to suppose that the membrana elastica externa has ceased to be developed (which in view of its extreme delicacy and unimportant function in Elasmobranchii is not difficult to do) and this cellular sheath would then obviously be homologous with the cartilaginous tube in question. In the Amniota an external sheath of the notochord cannot be traced as a distinct structure, but the connective tissue surrounding the notochord and spinal cord is simply differentiated into the vertebral bodies and vertebral arches.
Vertebral arches and Vertebral bodies.
Cyclostomata. The Cyclostomata are the most primitive forms in which true vertebral arches are present. Their ontogeny in this group has not been satisfactorily worked out. It is however noticeable in connection with them that they form for the most part isolated pieces of cartilage, the segmental arrangement of which is only imperfect.
Elasmobranchii. In the Elasmobranchii the cells forming the vertebral arches are derived from the splanchnic layer of the mesoblastic somites. They have at first the same segmentationas the somites (fig. 313,Vr), but this segmentation is soon lost, and there is formed round the notochord a continuous sheath of embryonic connective tissue cells, which gives rise to the arches of the vertebræ, the tissue forming the dura mater, the perichondrium, and the general investing connective tissue.
The changes which next follow result in what has been known since Remak as the secondary segmentation of the vertebral column. This segmentation, which occurs in all Vertebrata with true vertebræ, is essentially the segmentation of the continuous investment of the notochord and spinal cord into vertebral bodies and vertebral arches. It does not however follow the lines of the segmentation of the muscle-plates, but is so effected that the centres of the vertebral bodies are opposite the septa between the muscle-plates.
The explanation of this character in the segmentation is not difficult to find. The primary segmentation of the body is that of the muscle-plates, which were present in the primitive forms in which vertebræ had not appeared. As soon however as the notochordal sheath was required to be strong as well as flexible, it necessarily became divided into a series of segments.
The condition under which the lateral muscles can best cause the flexure of the vertebral column is clearly that each myotome shall be capable of acting on two vertebræ; and this condition can only be fulfilled when the myotomes are opposite the intervals between the vertebræ. For this reason, when the vertebræ became formed, their centres were opposite not the middle of the myotomes but the intermuscular septa.
These considerations fully explain the characters of the secondary segmentation of the vertebral column. On the other hand the primary segmentation (fig. 313) of the vertebral rudiments is clearly a remnant of a condition when no vertebral bodies were present; and has no greater morphological significance than the fact that the cells of the vertebræ were derived from the segmented muscle-plates, and then became fused into a continuous sheath around the notochord and nervous axis; till finally they became in still higher forms differentiated into vertebræ and their arches.
During the stage represented infig. 28g, and somewhat before the cartilaginous sheath of the notochord is formed, there appear four special concentrations of the mesoblastic tissue adjoining the notochord, two of them dorsal (neural) and two of them ventral (hæmal). They are not segmented, and form four ridges, seated on the sides of the notochord. They are unitedwith each other by a delicate layer of tissue, and constitute the substance in which the neural and hæmal arches subsequently become differentiated.
Illustration: Figure 318Fig. 318. Section through the vertebral column of an advanced embryo of Scyllium in the region of the tail.na.neural arch;ha.hæmal arch;ch.notochord;sh.inner sheath of notochord;ne.membrana elastic externa.
Fig. 318. Section through the vertebral column of an advanced embryo of Scyllium in the region of the tail.na.neural arch;ha.hæmal arch;ch.notochord;sh.inner sheath of notochord;ne.membrana elastic externa.
At about the time when the first traces of the cartilaginous sheath of the notochord arise, differentiations take place in the neural and hæmal ridges. In the neural ridge two sets of arches are formed for each myotome, and resting on the cartilaginous sheath of the notochord in the region which will afterwards form the centrum of a vertebra, and constituting a true neural arch; and a second separate from the cartilaginous sheath, forming an intercalated piece[200]. Both of them soon become hyaline cartilage.
There is a considerable portion of the original tissue of the neural ridge, especially in the immediate neighbourhood of the notochord, which is not employed in the formation of the neural arches. This tissue has a fibrous character and becomes converted into the perichondrium and other parts.
The hæmal arches are formed from the hæmal ridge in precisely the same way as the neural arches, but interhæmal intercalated pieces are often present. In the region of the tail the hæmal arches are continued into ventral processes which meet below, enclosing the aorta and caudal veins.
Since primitively the postanal gut was placed between the aorta and the caudal vein, the hæmal arches potentially invest a caudal section of the body cavity. In the trunk region they do not meet ventrally, but give support to the ribs. The structures just described are shewn in section infig. 318, in which the neural (na) and hæmal (ha) arches are shewn resting upon the cartilaginous sheath of the notochord.
While these changes are being effected in the arches the cartilaginous sheath of the notochord undergoes important differentiations. In thevertebralregions opposite the origin of the neural and hæmal arches (fig. 318) its outer part becomes hyaline cartilage, while the inner parts adjoining the notochord undergo a somewhat different development, the notochord in this part becomes at the same time somewhat constricted. In theintervertebralregions the cartilaginous sheath of the notochord becomes more definitely fibrous, while the notochord is in no way constricted. A diagrammatic longitudinal section through the vertebral column, while these changes are being effected, is shewn infig. 320B.
These processes are soon carried further. The notochord within the vertebral body becomes gradually constricted, especially in the median plane, till it is here reduced to a fibrous band, which gradually enlarges in either direction till it reaches its maximum thickness in the median plane of the intervertebral region. The hyaline cartilage of the vertebral region forms a vertebral body in which calcification may to some extent take place. The cartilage of the base of the arches gradually spreads over it, and on the absorption of the membrana elastica externa, which usually takes place long before the adult state is reached, the arch tissue becomes indistinguishably fused with that of the vertebral bodies, so that the latter are compound structures, partly formed of the primitive cartilaginous sheath, and partly of the tissue of the bases of the neural and hæmal arches. Owing to the beaded structure of the notochord the vertebral bodies take of necessity a biconcave hourglass-shaped form.
The intervertebral regions of the primitive sheath of the notochord form fibrous intervertebral ligaments enclosing the unconstricted intervertebral sections of the notochord.
A peculiar fact may here be noticed with reference to the formation of the vertebral bodies in the tail of Scyllium, Raja, and possibly other forms,viz.thatthere are double as many vertebral bodies as there are myotomes and spinal nerves. This is not due to a secondary segmentation of the vertebræ but, as I have satisfied myself by a study of the development, takes place when the vertebral bodies first become differentiated. The possibility of such a relation of parts is probably to be explained by the fact that the segmentation of the vertebral column arose subsequently to that of the nerves and myotomes.
Ganoidei. In Acipenser and other cartilaginous Ganoids the hæmal and neural arches are formed as in Elasmobranchii, and rest upon the outer sheath of the notochord. Since however the sheath of the notochord is never differentiated into distinct vertebræ, this primitive condition is retained through life.
Teleostei. In Teleostei the formation of the vertebral arches and bodies takes place in a manner, which can be reduced, except in certain minor points, to the same type as that of Elasmobranchii.
There are early formed (fig. 314kandk´) neural and hæmal arches resting upon the outer sheath of the notochord. The latter structure, which, as mentioned onp.549, corresponds to the cartilaginous sheath of the notochord of Elasmobranchii, soon becomes divided into vertebral and intervertebral regions. In the former ossification directly sets in without the sheath acquiring the character of hyaline cartilage (Götte, 419). The latter forms the fibrous intervertebral ligaments. The notochord exhibits vertebral constrictions.
The ossified outer sheath of the notochord forms but a small part of the permanent vertebræ. The remainder is derived partly from an ossification of the connective tissue surrounding the sheath, and partly from the bases of the arches, which do not spread round the primitive vertebral bodies as in Elasmobranchii. The ossifications in the tissue surrounding the sheath usually (fig. 319) take the form of a cross, while the bases of the arches (kandk´) remain as four cartilaginous radii between the limbs of the osseous cross. In some instances the bases of the arches also become ossified, and are then with difficulty distinguishable from the other parts of the secondary vertebral body. The parts of the arches outside the vertebral bodies are for the most part ossified (fig. 319). In correlation with the vertebral constrictions of the notochord the vertebral bodies are biconcave.
Amphibia. Of the forms of Amphibia so far studied embryologically the Salamandridæ present the most primitive type of formation of the vertebral column.
It has already been stated that in Amphibia there is presentaround the notochord a cellular sheath, equivalent to the cartilaginous sheath of Elasmobranchii. In the tissue on the dorsal side of this sheath a series of cartilaginous processes becomes formed. These processes are the commencing neural arches; and they rest on the cellular sheath of the notochord opposite the middle of the vertebral regions.
Illustration: Figure 319Fig. 319. Vertical section through the middle of a vertebra of Esox lucius (Pike).(From Gegenbaur.)ch.notochord;cs.notochordal sheath;k.andk´.cartilaginous tissue of the neural and hæmal arches;h.osseous hæmal process;n.spinal canal.
Fig. 319. Vertical section through the middle of a vertebra of Esox lucius (Pike).(From Gegenbaur.)ch.notochord;cs.notochordal sheath;k.andk´.cartilaginous tissue of the neural and hæmal arches;h.osseous hæmal process;n.spinal canal.
A superficial osseous layer becomes very early formed in each vertebral region of the cellular sheath; while in each of the intervertebral regions, which are considerably shorter than the vertebral, there is developed a ring-like cartilaginous thickening of the sheath, which projects inwards so as to constrict the notochord. At a period before this thickening has attained considerable dimensions the notochord becomes sufficiently constricted in the centre of each vertebral region to give a biconcave form to the vertebræ for a very short period of fœtal life.
The stage with biconcave vertebræ is retained through life in the Perennibranchiata and Gymnophiona.
The chief peculiarity which distinguishes the later history of their vertebral column from that of fishes consists in the immense development of theintervertebralthickenings just mentioned, which increase to such an extent as to reduce the notochord, where it passes through them, to a mere band; while the cartilage of which they are composed becomes differentiated into two regions, one belonging to the vertebra in front, the other to that behind, the hinder one being convex, and the anterior concave. The two parts are not however absolutely separated from each other.
By these changes each vertebra comes to be composed of (1) a thin osseous somewhat hourglass-shaped cylinder with a dilated portion of the notochord in its centre, and (2 and 3) of twohalves of two intervertebral cartilages,viz.an anterior convex half and a posterior concave half. The vertebræ thus come to be opisthocœlous. A longitudinal section through the vertebral column at this stage is diagrammatically shewn infig. 320C.
Illustration: Figure 320Fig. 320. Diagram representing the mode of development of the vertebræ in the different types.(From Gegenbaur.)A. Ideal type in which distinct vertebræ are not established.B. Type of Pisces with vertebral constrictions of the notochord.C. Amphibian type, with intervertebral constrictions of the notochord by the intervertebral parts of the cellular sheath.D. Intervertebral constriction of the notochord as effected in Reptilia and Aves.E. Vertebral constriction of the notochord as effected in Mammalia, the intervertebral parts of the cartilaginous sheath being converted into intervertebral ligaments.c.notochord;cs.cuticular sheath of notochord;s.cartilaginous sheath;v.vertebral regions;iv.intervertebral regions;g.intervertebral joints.
Fig. 320. Diagram representing the mode of development of the vertebræ in the different types.(From Gegenbaur.)A. Ideal type in which distinct vertebræ are not established.B. Type of Pisces with vertebral constrictions of the notochord.C. Amphibian type, with intervertebral constrictions of the notochord by the intervertebral parts of the cellular sheath.D. Intervertebral constriction of the notochord as effected in Reptilia and Aves.E. Vertebral constriction of the notochord as effected in Mammalia, the intervertebral parts of the cartilaginous sheath being converted into intervertebral ligaments.c.notochord;cs.cuticular sheath of notochord;s.cartilaginous sheath;v.vertebral regions;iv.intervertebral regions;g.intervertebral joints.
To the centre of each of these vertebræ the neural arches, the origin of which was described above, become in the meantime firmly attached; and grow obliquely upwards and backwards, so as to meet and unite above the spinal cord. The transverse processes of the vertebræ would seem (Fick) to be developed independently of the arches, though they very soon fuse with them. According to Götte the transverse processes are double in the trunk, there being two pairs, one vertically above the other for each vertebra. The pair on each side eventually fuse together.
In the tail hæmal arches are formed, which are similar in their mode of development to the neural arches.
The unconstricted portion of the notochord, which persists in each vertebra, becomes in part converted into cartilage.
Anura. In the Anura the process of formation of the vertebral column is essentially the same as that in the Salamandridæ. Two types may however be observed. One of these occurs in the majority of the Anura, and mainly differs from that in Salamandra in (1) the earlier fusion of the arches with the cellular sheath of the notochord; (2) the more rapid growth of the intervertebral thickenings of the cellular sheath, which results in the early and complete obliteration of the intervertebral parts of the notochord; (3) the complete division of these intervertebral thickenings into anterior and posterior portions, which unite with and form the articular surfaces of two contiguous vertebræ. The vertebræ are moreover procœlous instead of being opisthocœlous.
The unconstricted vertebral sections of the notochord always persist till the ossification of the vertebræ has taken place. In some forms they remain through life (Rana), while in other cases they eventually either wholly or partially disappear.
The second type of vertebral development is found in Bombinator, Pseudis, Pipa, and Pelobates. In these genera the formation of the vertebra takes place almost entirely on the dorsal side of the notochord; so that the latter forms a band on the ventral side of the vertebral column. In other respects the history of the vertebral column is the same in the two cases; the vertebral unconstricted parts of the notochord appear however to become in part converted into cartilage. The type of formation of the vertebral column in these genera has been distinguished as epichordal in contradistinction to the more normal or perichordal type.
Amniota. In the Amniota all trace of a distinction between a cellular notochord sheath and an arch tissue is lost, and the two are developed together as a continuous whole forming an unsegmented tube round the notochord, with a neural ridge which does not at first nearly invest the neural cord. This tube becomes differentiated, in the manner already described for other types, into (1) vertebral regions with true arches, and (2) intervertebral regions.
Reptilia. In Reptilia (Gegenbaur,No.416) a cartilaginous tube is formed round the notochord, which is continuous with the cartilaginous neural arches. The latter are placed in the vertebral regions, and in these regions ossification very early sets in, while the notochord remains relatively unconstricted. In the intervertebral regions the cartilage becomes thickened, as in Amphibia, and gradually constricts the notochord. The cartilage in each of the intervertebral regions soon becomes divided into two parts which form the articular faces of two contiguous vertebræ.
The general character of the vertebral column on the completion of these changes is shewn infig. 320D. The later changes are relatively unimportant. The constricted intervertebral sections of the notochord rapidly disappear, while the vertebral sections become partially converted into cartilage, and only cease to be distinguishable at a considerably later period.
The ossification extends from the bodies of the vertebræ into the arches and into the articular surfaces, so that the whole vertebræ eventually become ossified.
The Ascalabotæ (Geckos) present an exceptional type of vertebral column which has many of the characters of a developmental stage in other Lizards. The body of the vertebra is formed of a slightly hourglass-shaped osseous tube, united with adjoining vertebræ by a short intervertebral cartilage. There is a persistent and continuous notochord which, owing to the small development of the intervertebral cartilages, is narrower in the vertebral than in the intervertebral regions.
Aves. In Birds the cellular tube formed round the notochord is far thicker than in the Reptilia. It is continuous in the regions of the future vertebræ with neural arches, which do not at first nearly enclose the spinal cord.
On about the fifth day, in the case of the chick, it becomes differentiated into vertebral regions opposite the attachments of the neural arches, and intervertebral regions between them; the two sets of regions being only distinguished by their histological characters. Very shortly afterwards each intervertebral region becomes segmented into two parts, which respectively attach themselves to the contiguous vertebral regions. A part of each intervertebral region, immediately adjoining the notochord, does not however undergo this division, and afterwards gives rise to the ligamentum suspensorium.
The notochord during these changes at first remains indifferent, but subsequently, on about the seventh day in the chick, a slight constriction of each vertebral region takes place; so that the vertebræ have temporarily, as they have also in Amphibia, a biconcave form which repeats the permanent condition of most fishes. By the ninth and tenth days, however, this condition has completely disappeared, and in all the intervertebral portions the notochord has become distinctly constricted, and at the same time in each vertebral portion therehave also appeared two constrictions of the notochord giving rise to a central and to two terminal enlargements.
On the twelfth day the ossification of the cartilaginous centra commences.
The first vertebra to ossify is the second or third cervical, and the ossification gradually extends to those behind. It does not commence in the arches till somewhat later than in the bodies. For each arch there are two centres of ossification, one on each side.
The notochord persists for the greater part of fœtal life and even into post-fœtal life. The larger vertebral portions are often the first completely to vanish. They would seem in many cases at any rate (Gegenbaur) to be converted into cartilage, and so form an integral part of the permanent vertebræ. Rudiments of the intervertebral portions of the notochord may long be detected in the ligamenta suspensoria.
Illustration: Figure 321Fig. 321. Longitudinal section through the vertebral column of an eight weeks’ human embryo in the thoracic region.(From Kölliker.)v.cartilaginous vertebral body;li.intervertebral ligament;ch.notochord.
Fig. 321. Longitudinal section through the vertebral column of an eight weeks’ human embryo in the thoracic region.(From Kölliker.)v.cartilaginous vertebral body;li.intervertebral ligament;ch.notochord.
Schwarck (No.420) states that in both the intervertebral and the vertebral regions, though less conspicuously in the former, the cartilage is divided into two layers, aninnerand anouter. He holds that the inner layer corresponds to the cartilaginous notochordal sheath of the lower types, and the outer to the arch tissue. Ossification (Gegenbaur) of the centra appears in a special inner layer of cartilage, which is probably the same as the inner layer of the earlier stage, though this point has not been definitely established.
Mammalia. The early development of the perichordal cartilaginous tube and rudimentary neural arches is almost the same in Mammals as in Birds. The differentiation into vertebral and intervertebral regions is the same in both groups; but instead of becoming divided as in Reptilia and Birds into two segments attached to two adjoining vertebræ, the intervertebral regionsbecome in Mammals wholly converted into the intervertebral ligaments(fig. 322li). There are three centres of ossifications for each vertebra, two in the arch and one in the centrum.