I20 Elementary Zoology. 



as in the case of Amphioxus, or the embryo, if not hatched 

 until it is full grown, as in the earthworm and rabbit, has some 

 special means of nutrition independent of the yolk contained 

 in the ovum. The earthworm embryo lives upon the albumen 

 in the cocoon; the young rabbit is nourished by the mother 

 through the placenta. On the other hand, the animals hatched 

 out from eggs with abundant yolk are nourished during 

 growth by that yolk, and are born in a more or less adult 

 condition, as in the case of the chick. The frog is inter- 

 mediate; the larva is older when it leaves the egg than that 

 of the Amphioxus. Some frogs are not hatched as tadpoles, 

 but as frogs ; this is due to a larger and, consequently, more 

 yolk-laden ovum. 



If there were any doubt as to the unicellular character of 

 the ovum from an examination of its structure when mature, 

 this doubt would be entirely removed by the mode of develop- 

 ment of the ovum. 



The ova in the very immature ovary can be detected as 

 cells only a little larger than the other cells which form the 

 tissue of the ovary. The larger cells, destined to become ova, 

 are surrounded by a layer which ultimately becomes several 

 layers thick of the smaller, non-generative cells; this layer is 

 termed the follicle, and its cells contribute to the nourishment 

 of the egg cell. In certain animals processes of these cells 

 have been seen to grow out and come into contact with the 

 protoplasm of the ovum ; and it has even been asserted, though 

 the view is not generally accepted, that the yolk is actually 

 elaborated in these cells, and then, as it were, eaten by the ovum. 



COMPARISQN OF THE OvUM WITH AN ORDINARY TiSSUE CeLL 

 AND WITH THE SIMPLEST AnIMALS. 



It has at various times been attempted to be shown that 

 eggs in some cases are not single cells. It may, however, be 

 taken for granted that the ovum is invariably a single cell. 

 The main reasons for this conclusion as to the morphological 

 nature of the ovum are as follows : In the first place, the young 

 ova, in most cases, are perfectly indistinguishable from cells 



