In the Mammalian ovum 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 develop 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 uncommon. 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[24] no distinct investing protoplasmic layer or germinal disc is present till after impregnation, but the ovum is formed of a superficial layer with minute yolk-spherules, and of a central portion with larger yolk-spheres. [24] Kupffer, _Laichen u. Entwicklung des Ostsee-Härings_. Berlin, 1878. In Amphibia the yolk very often appears in the form of oval or quadrilateral 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 evidence, 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.