62 



CHORDATE ANATOMY 



proceed to develop directly into a tvpical vertebrate. But it does not. It 

 acquires no vertebral column; the notochord serves as definitive axial 

 skeleton. It develops no structures morphologically similar to the heart, 

 kidneys, specialized sense organs, or paired appendages of a vertebrate. 

 Further, in later development it acquires, especially in the head region, 



a variety of unique structures which 

 adapt the adult to its peculiar mode 

 of living but make it conspicuously 

 unlike any adult vertebrate. 

 Nevertheless Amphioxus is "verte- 

 brate" in too many features to 

 make it credible that they could 

 have arisen otherwise than in gene- 

 tic relationship with those of the 

 vertebrates. Herein, then, lies in 

 part the justification for describing 

 the early development of Amphi- 

 oxus to illustrate the main features 

 of the corresponding stages of 

 vertebrates. Further justification 

 is derived, as already stated, from 

 the fact that the paucity of yolk in 

 the egg of Amphioxus relieves the 

 embryo of the factor which in- 

 troduces varying degrees of com- 

 plication into the development of 

 vertebrates and occasions much 

 difficulty in the study and inter- 

 pretation of the processes. 



Organogenesis in the Vertebrates 



In the late embryo of Amphi- 

 oxus the main lines of the body plan 

 of a vertebrate are drawn. Brief 

 statements concerning the embry- 



D V 



Fig. 54. — 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 



plate; NT, neural tube; V, blood-vessel onic Origin of the major Organs of 



(paired dorsal aorta). vertebrates follow. 



Neural Tube. In Amphioxus the neural plate becomes detached 

 from the adjacent lateral ectoderm (Fig. 42) and transforms itself into a 

 tube not until after it has been covered by the lateral ectoderm. In 

 vertebrates a longitudinal folding of the neural plate and adjoining ecto- 

 derm occurs in such a way that the movement of the neural material 



