376 OUTLINES OF CHORDATE DEVELOPMENT 



corona radiata, mentioned above. These cells appear to be 

 directly connected with the ovum by fine pseudopodial proc- 

 esses affording the pathways by which substances enter the 

 ovum, providing for its growth. The other cells of the follicle, 

 form what is known as the stratum granulosum. 



As the follicle and egg approach maturity the follicular cavity, 

 containing the liquor folliculi, becomes very large, and the whole 

 structure becomes enclosed in a definite capsule consisting 

 externally of connective-tissue fibers and cells formed from the 

 stroma of the ovary, and internally of a thick layer of cells, 

 blood-vessels, and nerves. Within this lies the basement 

 membrane of the follicular epithelium (granulosa cells). At 

 the close of the oogonial growth period, the cells of the corona 

 radiata form a thick membrane (zona pellucida) around the 

 egg, and the protoplasmic processes remain only partially and 

 indistinctly (zona radiata). The full-grown follicle is very 

 large (9-14 mm. in man) forming a well-marked projection from 

 the surface of the ovary. The mammalian follicle is usually 

 known as the Graafian follicle. It was first described in 1677 

 by Regnerus de Graaf and was regarded as what we should 

 now call the ovum, until Von Baer's description in 1827 of 

 the true mammalian ovum. 



As to the history of the egg itself during the growth stage, 

 little need be said here. A differentiated region around the 

 nucleus appears very early, before the follicle has definitely 

 formed. This becomes a sort of "yolk nucleus' 7 giving rise 

 to the deutoplasmic content of the egg; it disappears while the 

 follicle is still single layered. During the growth period, which 

 is also a period of "organization" of the ovum, the nucleus 

 appears to give off chromatic substance into the cytoplasm. A 

 pair of small centrioles with surrounding centrosphere may be 

 seen during the early stages of growth. 



3. Maturation 



At the close of the growth period the nucleus forms a large 

 clear vesicle, with very little chromatin other than that of the 



