DEVELOPMENT OF THE GENERATIVE ORGANS FOI 
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 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. 
Pandistic and Meriistic Ova.—The ova of insects generally 
exhibit two distinct types, which have been described as 
PanGistic and Merdistic 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 
Merdistic 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, 
