294 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



take it as probable, for obvious mechanical reasons, that the rigid 

 skeletal masses arose in a position alternating with the muscle 

 segments. The individual vertebral centra were in other words 

 from the beginning intersegmental in position in relation to the 

 general body metamerism. 



In sketching out in somewhat greater detail the further develop- 

 ment of the vertebral column the assumption will be again made use 

 of, as it was in dealing with the mesoderm segments, that the trunk 

 region has in all probability departed least from the primitive con- 

 dition, and the facts quoted will in the main be taken from this 

 region of the body. 



The student who goes on to peruse original memoirs will notice 



that this rule is by no 

 A. B A. B. means always accepted. Some 



writers will be found to 

 assume that the caudal re- 

 gion is more nearly primitive, 

 and, in accordance with this 

 assumption, to interpret the 

 phenomena observed in the 

 trunk vertebrae by those 

 observed in the caudal, in- 

 stead of vice versa. 



In this connexion it must 

 be borne in mind that the 

 Vertebrate is above all 

 things essentially a coelomate 

 animal. No one doubts that 

 whatever the common ancestor of the Vertebrates was like it was 

 at least coelomate. And most morphologists would admit further 

 that the weight of evidence indicates that in this ancestor the 

 splanchnocoele extended throughout the greater part of its length 

 and that the existence of a considerable stretch of body towards the 

 hind end devoid of splanchnocoele (i.e. a tail region) is secondary. 

 But if the caudal region has in this way undergone profound 

 secondary modification of its structure it is clear that it is riot 

 in this region of the body that we should expect to find persisting 

 primitive modes of development of the axial skeleton. 



It is now necessary to follow but the fate of the arch-elements. 

 As already mentioned the primitive arrangement of these appears to 

 have been two pairs to each segment, above and below, so that corre- 

 sponding with each myotome there were, on each side, two neural 

 elements an anterior (A) and a posterior (B), and two haemal elements 

 an anterior (a) and a posterior (b). 



Neural Arches. 1 — Apparently the most nearly primitive arrange- 



1 In writing these sections on the vertebral column much use has been made of 

 Schauinsland's descriptions (1906) to which the student is referred for a more detailed 

 account than is here possible. 



Fig. 146. — Arrangement of dorsal arch-elements in 

 hinder trunk region of a Petromyzon larva 95 

 mm. in length. (After Schauinsland, 1906.) 



A, anterior, B, posterior neural arch-elements ; d.r, dorsal 

 root of spinal nerve ; N, surface of notochord ; v, interseg- 

 mental blood-vessel ; v.r, ventral root of spinal nerve. 



