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 



wy. 



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

 after Balfotjr. 



a. Connective-tissue capsule of the egg ; h, 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 ; y, space 

 originally occupied by the germinative vesicle, but made empty by its shriveUiug 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, 

 Pubkinje). 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 diflFer 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 /a in 

 diameter, which acquire a punctate appearance from the presence 

 of nunierous 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.— Tolk-elements from the Fowl's egg, after Balfoub. a, Yellow yolk ; b, white yolk. 



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

 spherules which efiect a transition between them. 



(fig. 8) has a different appearance 

 This results from the fact ihat 

 there is deposited around the j'olk, when it detaches itself from 



The freshly laid Hen's egg 

 from that of such an ovarian egg. 



