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 oogonium may 

 function as nurse cells and even neighboring oogonia 

 are engulfed by the oogonium 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 oogonium 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 oocytes 

 {y.o) and growing oocytes, and (3) epithelial cells. 

 The nurse cells and oocytes are both derived from the 

 oogonia ; 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 



