;o8 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 Panoistic 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. 



