GERM CELLS AND THEIR FORMATION 121 



Turning now to the formation of the sperm we find, as we 

 should expect, processes on the whole entirely comparable 

 with those of egg formation. The chief differences result from 

 the fact that in the ovary conditions are associated with the 

 formation of a few relatively large cells, while in the testis 

 small cells are formed, but in very large numbers. In Sponges 

 and Hydroids there is the same non-localized formation of the 

 sperm as of the ova, the germ cells being distinguished not so 

 much by position as by size. Apparently any ectoderm cell 

 may enlarge and become reproductive. In all forms above 

 these there are definite testes. Among many Coelenterates 

 and Echinoderms the testis is composed purely of germ cells, but 

 usually the testis, like the ovary, contains other interstitial or 

 accessory cells, and frequently these are directly nutritive in 

 function. The general structure of the testis differs from that 

 of the ovary in that its epithelium is thrown into folds, forming 

 either simple columns or acini, each with an efferent duct or 

 pathway which is to be considered coelomic in origin. In the 

 testicular epithelium, which is ordinarily reducible to the strati- 

 fied type, we find sperm cells in all stages of formation, from 

 spermatogonia to fully formed spermatozoa. As we have 

 already said, the process of sperm formation is usually continu- 

 ous, though frequently periodic (seasonal) in its rate or inten- 

 sity. When the testis is composed of acini, each is usually 

 surrounded by a follicle, equivalent in function to the egg folli- 

 cle. But when arranged in lobules and columns, each of which 

 may be derived from a single primordial cell or prespermatogon- 

 iunij the nutritive cells do not show this follicular arrangement, 

 but are fewer in number and scattered along the basement 

 membrane of the epithelium, or along the connective tissue 

 partitions bounding the spermatic columns of the lobule, and to 

 some extent among the germ cells proper. 



Among the Craniates the typical arrangement is that shown 

 in Figs. 68, 69. Here, along the basement membrane, are several 

 generations of spermatogonia with the scattered nutritive 

 basal cells, sometimes called also the Sertoli cells, usually larger 

 than the spermatogonia. As the spermatogonia increase in 



