66 



INTRODUCTION TO CLASSIFICATION. 



the outer, serous, or epidermic, layer rises up into a fold, which 

 encircles the whole embryo ; and the anterior and posterior parts 



Fig. 33. 



Fig. 33. Development of the Chick. 



A. First rudiment of the embryo; a, its cephalic; b, its caudal end ; c, primitive groove. 



B. The embryo further advanced ; a, b, c, as before; d, the dorsal lamina; developed in 



the cephalic region only, and nearly uniting in the middle line ; e, the proto-vertebrae. 



C. Letters as before. The dorsal laminae have united throughout the greater part of the 



cephalic region, and are beginning to unite in the anterior spinal region. 



D. Embryo further advanced (second day), the dorsal lamina; having united throughout 



nearly their whole length. The proto-vertebrsc have increased in number, and the 

 omphalo-meseraic veins, /, are visible. 



The embryos are drawn of the same absolute length, but it will be understood that the 

 older embryos are, in nature, longer than the younger. 



