HISTOLOGICAL DEMONSTRATIONS. 143 



dda ; the envelope of the cell. 5. The tunica granulosa 

 covering the zona. 6. The space that contains the liquor 

 folliculi placed outside the tunica. 7. The membrana 

 granulosa lining the ovisac. 8. The cumulus proligerus 

 uniting the tunica with the membrana. 5, 7, and 8 consist 

 of epithelial cells. 9. The ovisac composed of a fibro-vas- 

 cular tissue. 



255. Examine (H.) the section of an ovary of a young 

 kitten, showing here and there one of the epithelial cells of 

 the columnar epithelium on the peritoneal aspect of the 

 ovary enlarged. This, as described by Foulis, becomes the 

 nucleated protoplast of the future ovum. It becomes en- 

 closed by connective tissue corpuscles growing from the 

 stroma up around it. These eventually produce the cells 

 of the tunica and membrana granulosa, which for a long 

 time form one mass, but eventually become separated by 

 the appearance of the liquor folliculi. The zona pellucida 

 is an after formation, possibly a sort of excretion derived 

 from the protoplasm of the ovum, or from the cells of the 

 tunica granulosa. 



The tubes which have been described in the ovary 

 appear to be nothing more than sulci produced by the 

 unequal outward growth of the ovary (Foulis). Young ova 

 may be found growing on the surfaces of the sulci in the 

 same manner as on the free surface of the ovary. 



DEVELOPMENT. 



256. Examine a series of models of the human embryo, 

 and preparations of the embryo chick. A description of the 

 structure of the latter, and of the details of the methods 

 of preparation, would carry us beyond the limits of these 

 outlines. The advanced student will, however, find a com- 

 plete account of the subject in the Elements of Embryology, 

 by Foster and Balfour. (London: Macmillan, 1874.) 



