48 GRAN I AT A. 



in the ova of Amphibia, Marsipobranchii and Teleostei, and it attains 

 an immense development in the ova of Elasmobranchii, Reptilia, 

 and Aves. 



The food-yolk originates from a differentiation of the protoplasm 

 of the egg. It arises as a number of small highly refracting particles 

 in a stratum slightly below the surface. 



In the Mammalian ovurn these particles spread through the protoplasm 

 of the egg, but do not attain any considerable development. In other 

 forms the case is different. In Elasmobranch Fishes the refracting particles 

 appear to develope into vesicles, in the interior of which there arise 

 solid oval - or even rectangular highly refracting bodies, in the substance 

 of which a stratification may usually be observed, which gives them 

 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 developes 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). 



B A 



FIG. 2(J. YOLK ELEMENTS FKOM 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 evidence, to be related to the formation of the yolk. 



1 Knpffer, Laichcn u. Entwicklung des Oatsee-Harings. Berlin, 1878. 



