6O CRANIATA. 



an appearance not unlike that of striated muscle. In Teleostei the 

 yolk assumes very different characters in different cases. It is often 

 formed of larger or smaller vesicles containing in their interior other 

 bodies. Stratified plates like those of Elasmobranchii are also not un- 

 common. In the ripe ovum of Teleostei the food-yolk usually resolves 

 itself into a large vitelline sphere, which occupies the greater part of 

 the ovum, and is formed of a highly refracting fluid material which 

 coagulates on the addition of water. It contains in many instances one 

 or more highly refracting bodies known as oil globules, and is invested 

 by a granular protoplasmic layer continuous with the germinal disc, in 

 which a number of normal yolk-spherules are frequently present. In the 

 ovum of the Herring 1 no distinct investing protoplasmic layer or germinal 

 disc is present till after impregnation, but the ovum is formed of a super- 

 ficial layer with minute yolk-spherules, and of a central portion with larger 

 yolk-spheres. 



In Amphibia the yolk very often appears in the form of oval or quadri- 

 lateral plates. In Reptilia the yolk-spherules are vesicles, somewhat similar 

 to the white yolk-spheres of Aves, but as a rule without the highly refracting 

 spheres in their interior. The peculiar and complicated arrangement and 

 structure of the white and yellow yolk in Birds is fully described in the 

 " Elements of Embryology," and it need only be said that the yolk develops 

 in Birds in the same manner as in other types, and that at first all the yolk- 

 spherules appear in the form of white yolk. The yellow yolk-spheres are a 

 peculiar modification of white yolk-spheres, formed comparatively late in the 

 development of the egg (fig. 20).' 



FIG. 20. YOLK ELEMENTS FROM THE EGG OF THE FOWL. 

 A. Yellow yolk. B. White yolk. 



In the eggs of many Amphibia a dark granular mass known as the yolk 

 nucleus makes its appearance * and is supposed, without any very clear evi- 

 dence, to be related to the formation of the yolk. 



A body in the form of a shell enclosing a dark nucleus, which 

 is perhaps of the same nature, has been described by Eimer in the 

 Reptilian egg : it eventually resolves itself into a number of angular 

 fragments. In Elasmobranchii a similar body is perhaps present. 



The food-yolk just described is imbedded in the active proto- 

 plasmic portion of the body of the ovum. In the case of the 



1 Kupffer, Laichen u. Entwicklung des Ostsee-Harings. Berlin, 1878. 



