294 EMBRYOLOGY OF THE LOWER VERTEBRATES n. 



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 intersegniental 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 A. B. means always accepted. Some 



if fi tt . . ii i_ " n T 



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 



,o,t or spinal nerve ; K, surface of notochord ; *, inter**- ^ essentially a COelomate 



iin-iital l>lo<*l-v'ssel ; v.r, ventral root of spinal nerve. . o J 1 



animal. JSo 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 splanclmocoele (i.e. a tail region) is secondary. 

 P.m. if the caudal region has in this way undergone profound 

 secondary modification of its structure it is clear that it is not 

 in this re-inn of the body that we should expect to iind persisting 

 primitive modes of development of the axial skeleton. 



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

 i\ mentioned the primitive arrangement of these appears to 

 have heei i i w< pairs to each segment, above and hel..\\, so that eon-e 



*pondin;j with each mvntnme there were, on each side, t\\<> neural 

 elements aO anterior A _, and a posterior p, ,-md two liaemal elements 

 an anterior // .,ml a pOftteripT (I). 



NI-.I I;AI. Ai;< BBS. 1 Apparent Iv t he most nearly pri mil ive arrange- 



l'i<;. 146. Arrangement of dorsal arch -elements in 

 hinder trunk region of a Petromyzon larva 95 

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



A, anterior, B, ]>ostf'riorin>unilarch-flcine7its ; d.?-, dorsal 



1 In writing tli-si: '<-li<.iih .n tlir '. < oliiinli nilicli i. Bfl ni:il<- !' 



S.'liail: ulnrh (In- -tinlriil i- Idrllvd lor ;i limlr drt Jllli-il 



ible. 



