128 VERTEBRATA. 



The development of the embryo (fig. 580) begins with a total 01 

 partial (discoidal) segmentation. The first rudiment of the embryo 

 is usually a germinal disc or blastoderm lying upon the yolk. From 

 the posterior end of this disc the alimentary cavity is developed. A 

 primitive streak which marks the long axis of the embryo is developed 

 by a thickening of the layers of the blastoderm. Two laterally placed 

 longitudinal folds give rise to an ectodermal groove the medullary 

 groove or first rudiment of the central nervous system beneath 

 which is placed the notochord which is developed from the endoderm 

 (fig. 581). 



The medullary groove which is dilated anteriorly is closed by the 

 growing together of its edges, and the tube so formed gives rise to 

 the spinal cord and to the brain. Its lumen is for some time in 



Mp 



Mv 



FIG. 582. Transverse section through a chick embryo of the second day (after Kolliker). 

 EC, ectoderm ; N, medullary canal ; End, endoderm ; Ch, notochord ; UW, proto- 

 vertebra ; UNg, Wolffian duct (primitive duct of kidney); Mp, somatic mesoblast ; Mv, 

 splanchnic mesoblast; Lk, body cavity ; Ao, primitive aorta. 



communication with the alimentary cavity by the neurenteric canal. 

 At the sides of these structures the mesoderm extends in the form 

 of two bands, the median portions of which (protovertebral plates) 

 become segmented in the course of the further development and 

 give rise to the protovertebrse (figs. 576 and 582). The archinephric 

 duct is separated off at the boundary between the protovertebrae and 

 the unsegmented lateral plates of mesoblast, while the generative 

 glands arise nearer the median line from the peritoneum of the lateral 

 plates of mesoblast. While the dorsal part of the embryo is thus 

 being formed the alimentary canal becomes further developed on the 

 ventral side of the blastoderm, and gradually absorbs the yolk, often 

 leaving an external yolk sac. > The young animals only undergo 

 a metamorphosis in the naked Amphibia and several Fishes. 



The division of the Vertebrata into the four classes of Fishes, 

 Amphibia, Birds, and Mammals was first established by Linnaeus, 

 though it had been already indicated in the system of Aristotle. 



The Fishes and Amphibia are cold-blooded animals (i.e., animals 

 with a varying temperature) ; Aves and Mammals are warm-blooded 



