86 



COMPARATIVE ANATOMY 



-NP-^_, 



part of the tube in all vertebrate embryos. Later subdivision of the first 

 and third vesicles results in the live brain regions universally characteristic 

 of adult vertebrates. The nervous structures (retina and optic nerve) 

 of the paired eye grow out from the second (numbered from in front) 



region but the lens of the eye is 

 derived from neighboring super- 

 ficial ectoderm (Fig. 60). The 

 receptor (that is, stimulus-receiv- 

 ing) nervous structures of the ear 

 and olfactory organ originate not 

 from the neural tube but from 

 superficial ectoderm. 



Notochord. In contrast to the 

 quite definite manner of origin of 

 the notochord in Amphioxus, there 

 is considerable obscurity as to the 

 precise nature of the process in 

 many vertebrates and it must be 

 admitted that the process differs 

 markedly in the several classes. 

 In amphibians the notochord is in 

 part and to greater or less extent 

 folded off or split off from the mid- 

 dorsal endoderm and in part built 

 forward from the actively growing 

 blastoporal region. In amniotes 

 the origin of the notochord is closely 

 related to that of the mesoderm — 

 so much so that the notochord has 

 sometimes been described as of 

 mesodermal nature. Its material, 

 like that of the mesoderm, usually 

 seems to be derived from the 

 primitive streak (see pages 76, 78), 



As 



V 



Fig. 58. — Diagrams illustrating method 

 of origin of the neural tube of vertebrates. 

 Transverse sections in the mid-trunk region 

 of embryos at successively (A to D) later 

 stages. C, neural crest; CC, canalis 

 centralis of neural tube; EC, ectoderm; 

 EN, endoderm; MES, mesoderm; NC, 



notochord; NG, neural groove; NP, neural a region where ectoderm and endo 



plate; NT, neural tube; F, blood vessel j . ,. . • u ui 



(paired dorsal aorta). derm merge mdistmguishably 



cells proliferated from the streak 

 laterally give rise to mesoderm, so proliferation forward from the 

 anterior end of the streak produces a median cord of cells which form 

 the notochord. It may, however, receive accessions from the endoderm 

 with which it is usually in close relation. 



If it be granted that Amphioxus is a near relative of immediate ances- 

 tors of vertebrates, the mode of origin of its notochord does much to 



