16 



EMBRYOLOGY. 



quantity ; it forms a thin layer over the whole surface, the white- 

 yolk-rind ; secondly, it is accumulated in somewhat greater quantity 

 under the germinative vesicle, for which it at the same time forms a. 

 bed or cushion (PANDER'S nucleus) ; and, thirdly, from this region it 



Fig. 6b. Section of the germ-disc of a mature ovarian Hen's egg still enclosed in the capsule, 

 after BALFOUR. 



n, Connective-tissue capsule of the egg ; 6, epithelium of the capsule, on the inside of which lies 

 the vitelline membrane reposing upon the egg ; c, granular substance of the germinative 

 disc ; w.y, white yolk, which passes imperceptibly into the finely granular substance of the 

 disc ; a-, germinative vesicle enclosed in a distinct membrane, but shrivelled up : .". space 

 originally occupied by the germinative vesicle, but ma-le empty by its shrivelling up. 



penetrates in the form of a mortar-pestle into the very centre of the- 

 yellow yolk, where it terminates in a knob-like swelling (latebra., 

 PURKINJE). Upon boiling the egg, it is less coagulated, and remains- 

 softer than the yellow yolk. In the coagulated condition the latter 

 discloses upon sections a lamellated condition, in that it consists of 

 smaller and larger spherical shells, which envelope the latebra. 



The two kinds of yolk also differ from each other in respect to* 

 the condition of their elementary particles. The yellow yolk 

 consists of soft plastic spherules (fig. 7 A) from 25 to 100 /u, in 

 diameter, which acquire a punctate appearance from the presence 

 of numerous exceedingly minute granules. The elements of the 

 white yolk are for the most part smaller (fig. 7 B), and likewise 

 spherical, but contain one or several large highly refractive granules*. 



Tig. 7. Yolk-elements from the Fowl's egg, after BAI.FOUR. A, Yellow yolk ; B, white ydk. 



At the boundary Iw-twiui the two kinds of yolk there are present 

 spherules which < flfi ct a transition between them. 



The freshly laid Hen's egg (fig. 8) has a different appi aram-e 

 from that of such an ovarian egg. This results from the fact iliat 

 there is deposit rd around th<-> volk, when it detaches itself from 



