THE GERM CELLS 



For the present it is sufficient to state that each growing ovum is at first 

 surrounded by small nutritive cells known as follicle cells. These increase 

 in number during the growth of the ovum until several layers are formed 

 (Fig. 229). A cavity appearing between these cells becomes filled with 



U'.V. U'.V. fr I. 



M. 



FIG. 3. Diagrammatic longitudinal section of an unincubated hen's egg (Allen Thomson 

 in Heisler). bl., Blastoderm; w.y., white yolk, which consists of a central flask-shaped mass, and 

 of concentric layers alternating with the yellow yolk (y.y.); vt., vitelline membrane; to., al- 

 bumen; M., chalaza; a.ch., air chamber; i.s.m., inner, s.m., outer layer of the shell membrane; 

 s, shell. 



FIG. 4. Section of human ovary (Piersol). a. Germinal epithelium; 6, tunica albuginea; 

 c, cortical stroma containing immature follicles, d; e, shrinkage space between theca and 

 stratum granulosum of a well-advanced Graafian follicle; /, liquor folliculi; g, ovum surrounded 

 by cumulus oophorus. 



fluid and thus forms a sac, the vesicular (Graafian) follicle, within which 

 the ovum is eccentrically located (Figs. 4 and 230). The follicular cells 

 immediately surrounding the ovum form the corona radiata (Fig. i) 

 when the ovum is set free. 



