THE REPRODUCTIVE ORGANS 71 



nurse-cells to the egg (Figs. 40 and 41). The 

 thickness of the stratum of cells depends upon 

 the degree of development at which the ovum 

 has arrived. A theca composed of spindle- 

 shaped cells, consisting of a condensation of the 

 connective-tissue stroma of the ovary, encloses 

 the egg - follicle. The general ovarian stroma 

 is somewhat scanty, and highly vascular, 

 especially during the egg-laying season. 



The ovary is so surrounded by other organs 

 as to be enclosed in what Miss Curtis calls 

 an " ovarian pocket," the only way out of 

 which is by the infundibulum of the oviduct. 

 An egg, after escaping from a follicle of the 

 ovary, therefore, must of necessity pass into 

 the oviduct. The dorsal wall of the " pocket " 

 is formed by the body wall to which the ovary 

 is attached. The other boundaries of the 

 " pocket " are formed by the left abdominal 

 air -sac, a part of the intestine and the 

 mesentery. 



The Egg. — The fully formed egg (Fig. 42), as 

 one knows it after it has been laid and before 

 it has been incubated, is covered externally by 

 a porous shell in which three layers have been 

 distinguished. The innermost or mammillary 

 layer consists of minute conical masses of 



