92 



EMBEYOLOaY. 



treatment. In Birds and Reptiles the investigation is accompanied 

 witli greater difficulties than in the Selachians. Particularly the 

 development of the germ-layers in the Chick, notwithstanding the 

 fact that the best investigators have given it their attention, has 

 for a long time been the subject of very divergent descriptions. At 

 the present moment, however, the main facts in the case have been 

 established for the Bird's egg also by the very recent and excellent 

 work of Duval, and upon this as a basis the gastrulation in Birds is 

 easily to be correlated with that of the Vertebrates hitherto described. 

 Since the Bu'd's egg has played such an important r61e in the history 

 of embryology, and has even been called a classical object for investiga- 

 tion, it appears necessary to go briefly into the conditions which it 

 presents in the gastrula-stage, and in connection therewith to consider 

 some of the iniportant results drawn from the study of the eggs of 

 Reptiles. 



The blastula arises and the germ-layers begin to be developed out 

 of it whUe the Bird's egg tarries in the terminal region of the 

 oviduct. 



The blastula arises in a manner which was first correctly described 

 by Duval. When by the process of segmentation a small disc of 



cells has been formedj 

 there appears in the 

 latter a narrow fissure, 

 the cleavage-cavity (fig. 

 51 fh), and the cell- 

 matei'ial is separated 

 into an upper layer {dw) . 

 and a lower layer {vw), 

 which are continuous 

 with each other at the 

 margin of the disc. The 

 upper layer consists of 

 fully isolated cleavage- 

 spheres, which are flattened at their surfaces of contact and arranged 

 into an epithelium-like layer. They correspond to the thin-walled 

 half of the blastula in Triton (fig. 45), which has already been 

 designated as the animal half. The lower layer; is composed of 

 larger cleavage-spheres, which are still in great part continuous 

 by means of their lower halves with the white yolk (wd), which 

 is spread out beneath the germ-disc and is known as Pander's 

 nucleus. Yolk-nuclei (merocytes) are also found here in n-reat 



Tiff. 51.— Section througli the germ-disc of a freshly laid 



"t^ertilised Hen's e^g, after Duval. 

 fh, Oeavage-cavity ; wd, white yolk ; vw, lower ceU-layer ; 



dw, upper cell-layer of the blastula. 



