DEVELOPMENT OF THE GENERATIVE ORGANS 701 



to occupy the space which, in the earlier period of development, 

 is formed between the ovarian capsule and the epithelial layer, 

 from which the egg-stocks spring. 



Development of the Ova. The lowest rudimentary ovum in the 

 egg-stocks is seen to be in different stages of development in 

 the different egg-stocks. The chorion of the young eggs is a 

 cellular layer derived apparently from the upward growth of 

 the egg-stalk. In some of the ova it is seen as a cup partially 

 enclosing a number of small yelk-cells. These cells are about 

 7 M in diameter. This chorionic cup ultimately encloses from 

 ten to twelve yelk-cells, and then begins to form a second cup 

 around a second group of similar cells. 



The young yelk-cells enclosed in the chorion measure about 

 10 ^ in diameter, and become polyhedral by mutual pressure. 

 These cells then grow rapidly; one of them, that nearest to 

 the egg-stalk, however, soon becomes far larger than the others. 

 There is at first no difference in the nuclei of these cells, and 

 in the later stages of development the only difference which 

 can be observed in the yelk-cells is a difference of size. 



Panbistic and Meroistic Ova. The ova of insects generally 

 exhibit two distinct types, which have been described as 

 Panoistic and Meroistic ova. In some Insects, as in the 

 Orthoptera, the vitellus consists from the first of only a single 

 yelk-cell, enclosed in an epithelial chorion ; in the second form 

 several cells are enclosed within the chorion as in the Blow-fly. 

 This condition is common to all the Diptera, and is very general 

 in the Insecta. 



The Egg-cell. Brandt termed the larger basal cell of the 

 Meroistic ovum the egg-cell, and the remainder of the cells 

 nutrient cells. In the Blow-fly at least the cells are all at first 

 precisely similar, except that one appears to possess a greater 

 power of growth than the others ; but, as will be seen hereafter, 

 all these cells undergo a precisely similar series of changes, and 

 are ultimately broken down and form the granular yelk already 

 described (p. 683). 



The part played by the so-called nutrient cells is a subject 

 on which there is a great divergence of opinion. Brandt's view, 



