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 BALFOOR. 



a, Connective-tissue capsule of the egg ; b, 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 ; x, germinative vesicle enclosed in a distinct membrane, but shrivelled up ; y, space 

 originally occupied by the germinative vesicle, but made 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 p. 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. 



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



At the boundary between the two kinds of yolk there are present 

 spherules which effect a transition between them. 



The freshly laid Hen's egg (fig. 8) has a different appearance 

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

 there is deposited around the yolk, when it detaches itself from 



