THE GERM CELLS. 



is opened owing to the slightly higher specific gravity of the yolk. At the 

 time of laying, however, development has proceeded for several hours, for 

 fertilization normally occurs in the oviduct before the secondary egg-mem- 

 branes are deposited. The ovum proper must be examined in the ovary or 

 immediately after its escape therefrom in order to see it before development 

 begins. At this time the yolk mass is quite similar to that of the egg after 

 laying, and the small disk of cytoplasm containing a single flat nucleus is 

 attached to one side of the yolk. While a few small yolk granules are sus- 

 pended in the cytoplasm, there is an abrupt transition from the cytoplasmic 

 disk to pure yolk. By far the greater part of the yolk contains no cytoplasm 

 but consists solely of nutritive substances which are later carried to and 

 assimilated by the growing embryo. 



Germinal disk (cytoplasm) 



White yolk 



Albumen (" white 



Vitelline membrane 



White yolk 



Shell 



Shell membrane 

 (outer layer) 



Chalaza 



Shell membrane 

 (inner layer) 



Yellow yolk (deutoplasm) 

 FIG. 3. Diagram of a vertical section through an unfertilized hen's egg. Bonnet. 



The presence of the large quantity of yolk in the ova of birds and reptiles 

 is correlated with the long period during which embryos of these animals 

 undergo development within their shells before hatching and attaining 

 ability to get their own food. In the case of the frog the moderate amount 

 of yolk in the egg serves as food for the growing embryo until it becomes a 

 free-swimming larva or tadpole. An embryo of a mammal develops for a 

 long period in the uterus of its mother from an ovum with scanty yolk, but 

 provision is made for drawing nourishment directly from the maternal blood 

 during this time. 



A simple classification of ova is made on the basis of the amount and 

 distribution of the yolk content. The term meiolecithal is used, to designate 

 ova in which the yolk granules are few (many invertebrates, Amphioxus, 

 mammals). Mesolecithal ova are those which contain moderate quantities 

 of yolk (amphibians). Ova that possess large yolk content are classed as 



