314 ZOOLOGY FOR MEDICAL STUDENTS chap. 



movements up the cavity of the oviducts, and the actual process of 

 syngamy takes place towards the upper end of the tube before the egg 

 has become enclosed in its shell. 



The skeleton of the vertebrate is, as already indicated, an internal 

 skeleton, and we can recognize within the phylum three distinct phases 

 in its evolution which may be distinguished as (I) the notochordal, 

 (II) the cartilaginous, and (III) the osseous or bony phase. Of these three 

 phases the first is found in the Dogfish, as in the majority of vertebrates, 

 only in the developing embryo. It is represented by a simple elastic 

 rod^the notochord — lying along the dorsal side of the body immediately 

 ventral to the central nervous system, and extending from just behind 

 the tip of the infundibulum back practically to the tip of the tail. The 

 notochord is of the greatest possible morphological importance, for the 

 skeleton of every vertebrate, even the highest and most complicated, 

 passes through a stage in which it consists of a simple notochord. 



The notochord is endodermal in origin and is at first a simple rod of 

 endodermal cells (Fig. 131, A) separated off from the dorsal wall of the 

 embryonic alimentary canal. The superficial cells of this rod form on 

 their surface a strong cuticular membrane — ^the primary sheath (^.I) — 

 which invests the notochord from end to end. As the embryo develops ■ 

 the notochord increases actively in length and in thickness, and this 

 growth is brought about mainly by the great increase in volume 

 of its constituent cells, due to their developing large vacuoles in 

 their interior. This does not apply, however, to all the cells, for those 

 on the surface of the notochord remain unvacuolated, forming a layer of 

 notoehordal epithelium (Fig. 131, B, n.e). These cells continue to form 

 cuticular substance but now, instead of being thin and membranous, this 

 is thick and jelly-like and is known as the secondary sheath (C, ^.11). 



There now makes its appearance a new type of skeletal tissue — 

 cartilage (see p. 292) — which is destined to form the whole of the com- 

 plicated fully developed skeleton with the exception of the placoid scales 

 and the notochord. 



The spinal cord, which as has already been indicated lies immediately 

 dorsal to the notochord (Fig. 132), is covered over by a kind of tunnel 

 of connective tissue (Fig. 131, C, c.t^ : in the tail-region a similar tunnel, 

 but in a reversed position with its apex downwards, shelters the great 

 blood-vessels — the caudal artery and vein. It is in the walls of these 

 tunnels that the cartilage first makes its appearance in the form of 

 paired blocks in immediate proximity to the sheath of the notochord. 

 These blocks form the rudiments of what are known as the neural arches 



