ACCOUNT OF THE GERM-CELL CYCLE 37 
liquids and no special nurse cells are required; but 
larger eggs either become surrounded by follicle cells 
which nourish them and with which they are often 
intimately connected by protoplasmic bridges, or 
special nurse cells are provided. In the primitive 
type of ovary, such as exists in most ccelenterates, 
any of the cells surrounding the odgonium may 
function as nurse cells and even neighboring odgonia 
are engulfed by the odgonium that is successful in the 
struggle for development. A more definite mechan- 
ism exists in higher organisms, where one or more 
cells become differentiated for the special purpose of 
supplying nutriment consisting of either their own 
substance or of material elaborated by them and 
then transferred to the egg. The egg of the annelid, 
Ophryotrocha, for example, is accompanied by a single 
nurse cell; that of Myzostoma is provided with two, 
one at either end; and the eggs of certain insects 
are more or less intimately connected with groups of 
cells in definite nurse chambers (Fig. 46). 
The growth of an odgonium may be well illus- 
trated by that of the potato beetle. 
The general arrangement of the cells in the ovary 
of an adult beetle is shown in Fig. 7. The terminal 
chamber of the ovarian tubule contains three kinds 
of cells: (1) nurse cells (n.c), (2) young odcytes 
(y.o) and growing odcytes, and (3) epithelial cells. 
The nurse cells and odcytes are both derived from the 
odgonia; the epithelial cells are of mesodermal origin. 
The positions of the stages to be described are 
indicated in the diagram (Fig. 7) and the nuclear 
