FEMALE REPRODUCTIVE CELLS 



457 



other foreign bodies. These are described by Eigenmann as being 

 formed between the follicle cells. 



A definite step in the organization of such a layer of nourishing cells 

 is met with in the insects, where the egg may be said to be covered by a 

 single layer of nurse cells, one or 

 more of which is enlarged to per- 

 form the function of yolk storage. 

 Where several cells out of the layer 

 assume this role, they may con- 

 tinue to lie in a single layer or 

 they may become arranged in a 

 mass that is practically a stratified 

 epithelium. A good subject to 

 study is a "ground hornet," Sco- 

 lia dubia. In this insect the ova- 

 rian tubules are terminated dis- 

 tally by chambers of primordial 

 cells. A succession of single ova, 

 each surrounded by a layer of 

 follicle cells, arises from this ter- 

 minal chamber and moves with 

 the entire tubule toward the egg 

 duct as an egg-follicle. Alternating 

 with the egg-follicles are masses 

 of nurse cells surrounded by a 

 follicle epithelium to form a nurse- 

 cell follicle. Each nurse-cell fol- 

 licle is attached to its egg-follicle 

 on its distal end (Fig. 421). The ovum sends a cytoplasmic process 

 up into the fundus of the nurse-cell follicle. This latter follicle has in- 

 creased in size by the growth of the nurse cells. As the nurse cells grow, 

 their nuclei become irregular in contour and the chromatin breaks up 

 into minute granules which are uniformly distributed throughout the 

 nucleus. The cells which are first to so differentiate lie next the ovum 

 (Fig. 422). This differentiation results in the secretory powers of the 

 cells becoming active. A vacuole now appears in the egg cytoplasm, as 

 though a streaming fluid had excavated the egg in this region (Fig. 422, 

 vac.}. The growth of ovum and nurse-cell follicle continues. Even- 

 tually a wave of cytoplasmic disintegration passes with some regularity 

 through the nurse-cell follicle from the ovum distally. This is followed 

 by the disintegration of the nuclei. 



The destruction of the contents of the nurse-cell follicle causes it to 

 contract and finally disappear as an appendage of the egg-follicle. Fig- 



FIG. 421. Young egg-follicle and attached 

 nurse-cell follicle of the ground-hornet, Scolia 

 dubia. ov., ovum surrounded by a single layer 

 of passive (?) follicle cells; ./., follicle com- 

 posed of young active nurse cells which were 

 derived from a part of the single layer. X 350. 



