708 THE GENERATIVE ORGANS. 
Very generally, however, a certain amount of food material, 
deuteroplasm or food yelk, is seen embedded in the cell sub- 
stance, usually leaving the original protoplasm clear in the 
vicinity of the germinal vesicle. When this food substance is 
abundant, as in Birds, Reptiles, and some Fishes, the ova 
attain enormous dimensions. The food yelk then serves as a 
store of food material for the development of the young. 
This accumulated food material apparently interferes with the 
process of segmentation, which in extreme cases is confined 
to the clear protoplasm in the vicinity of the nucleus. Such 
ova are said to be meroblastic. 
The secondary or food yelk is admitted to result from the 
activity of the cells which surround the ovum; but opinions 
differ as to whether it is transferred from the nutrient cells to 
the egg-cells, or whether the former are broken up into the 
yelk which surrounds the latter. The first of these alterna- 
tives is more usually accepted in the case of oviparous 
Vertebrates, 
In certain Worms, however, it is indubitable that the yelk 
has a separate origin, and at a later period the germ yelk is 
imbedded within the food yelk. ° 
Amongst the Platyhelminthes, Cestodes, Trematodes, and 
most Turbellaria, the secondary yelk and the germ ova are 
developed in distinct organs, termed yelk-glands and germ- 
glands. In these animals the two elements are united in the 
uterus or oviduct, and there receive a common investment. 
The Ova of the Insecta.—The great yelks developed in the 
ovary are admitted to be composed, for the most part at least, 
of food or secondary yelk ; and a vast number of investigators 
have sought to demonstrate a germ yelk or germinal vesicle in 
these eggs. 
In the case of the Pandistic ova it is easy to suppose that 
the large nucleus of the yelk-cell is a germinal vesicle, and, 
although it differs entirely in appearance from the germinal 
vesicle of any other group of animals, many have been satis- 
fied that it is the germinal vesicle which afterwards becomes 
surrounded by an abundant secondary yelk. 
