CHAP, x.} THE DEVELOPMENT OF THE CAT. 



355 



cellular investment, or follicle, around the primordial ova, and 

 ultimately grow into the tunica granulosa within the Graafian 

 vesicle. Sometimes two or more of the ova-forming epithelial cells 

 of a nest, will coalesce and form a large protoplasmic mass with two 

 or more nuclei, and thus a cell may arise and then he nourished hy 

 appropriating the substance of other cells. An enlarged epithelial 



Fig. 159.- PORTION OF THE OVARY OF A KITTEN ABOUT THREE WEEKS OLD, SHOWING THE GERM- 

 EPITHELIUM AND PRIMORDIAL OVA, WITH LARGE EGG-NESTS MORE OR LESS SUR- 

 ROUNDED BY STROMA (Foulis). 



a. Primordial ova. 



I. Smaller cells, resulting from the fission of larger 



epithelial cells, and beginning to form the 



membrana granulosa. 



c. Large clusters of germ epithelial cells, i.e., 



large egg-nests. 



d. Large germ epithelial cells. 

 c. Stroma of ovary. 



/. Germ epithelial cells. 



cell, or primordial ovum, when it has become enclosed in a follicle, 

 becomes a " permanent ovum/' The protoplasm, or vitellus, accu- 

 mulates about the nucleus or germinal vesicle, which developes a 

 nucleolus, or spot, while the zona pellucida forms either just within or 

 externally to a delicate membrane investing the vitellus. The ovum, 

 with its epithelial lining and external investment of fibrous vascular 

 stroma, is a " Graafian follicle." At first the ovum itself is closely 

 embraced by the follicle. Afterwards the follicle greatly enlarges, 

 and its contents separate into (1) the cells of the membrana granulosa, 

 lining the vesicle and enclosing (2) a fluid, (3) the ovum, and (4) an 

 aggregation of cells the discus proligerus which remains adherent 

 around the ovum. The ovary at first contains thousands of incipient 

 ova, but the great majority disappear as development proceeds. When 

 the kitten is but from two to four weeks old a very large part of the 

 whole ovary consists of egg-clusters, but at five months old there is only 

 a zone of young eggs left immediately beneath the tunica albuginea.* 

 17. The NERVOUS SYSTEM, though a unity in its fully formed 

 condition, is more diverse in its origin than is the alimentary 

 system. The latter system, as has been shown, arises as a groove 



* See Dr, Foulis, I. c., p. 373. 



A A 2 



