GERM CELLS AND THEIR FORMATION 



111 



eral the frame-work of the gonad, have been formed in situ 

 from the proliferating peritoneal cells. Others, the primordial 

 germ cells, are often first distinguishable in some other region of 

 the developing embryo (Fig. 59, A); they then make their way 

 into this germinal epithelium as development proceeds. 



The gonad may consist almost entirely of the reproductive 

 cells proper, and may be then a more or less periodic structure, 



FIG. 58. A. Part of a section through the body of a young lizard, Lacerta 

 agilis, showing the genital ridges and associated structures. B. Genital ridge, 

 enlarged, showing young follicles containing ova. From Korschelt and Heider, 

 after Braun. ao, dorsal aorta; V, cardinal vein; ms, mesentery. 



almost if not quite disappearing between the periods of repro- 

 ductive activity. Or i.t may be a permanent organ of con- 

 siderable though varying size, consisting of a complex stroma 

 of a variety of cells nutritive, vascular, nervous, connective 

 tissue, and other cells, in addition to the true germ cells. When 

 highly developed the gonad may also contain various cavities, 

 of ccelomic character, into which the germ cells are passed when 

 ripe. Thence they may pass directly into the body cavity 

 from which exit is made to the outside through simple perfora- 



