18 HARLEY N. GOULD 



The gonad is now a small, ramifying body, lying between the 

 alimentary tract and the digestive gland, and sometimes extend- 

 ing between the lobes of the latter. This condition may be re- 

 tained for a long time. As has been said, 'sexually inactive' 

 animals are often found in which the gonad is in an entirely un- 

 developed condition. Some animals have been kept in the 

 aquaria for long periods without showing any signs of male de- 

 velopment (e.g., outgrowth of penis), and when killed and sec- 

 tioned they showed only a rudimentary gonad; while others 

 during the same time became adult males. 



Male development is inaugurated by the rapid division of the 

 primordial male cells, cutting off spermatogonia which lie free in 

 the lumen; and by the enlargement of the whole organ. The 

 spermatogonial cells continue to divide rapidly, if male develop- 

 ment goes on without interruption, and the lumen becomes solidly 

 filled with them. The rapidly enlarging gonad, following the 

 lines of least resistance, puts out new extensions which become 

 swollen into follicles reaching in all directions. The ultimate 

 shape of the gonad depends upon the position of the surrounding 

 organs. The pressure of the latter affects the gonad differently 

 in different animals; for the testis is of varying shape and exten- 

 sion. It sometimes reaches far forward, sometimes far back- 

 ward toward the posterior end of the visceral sac, sometimes 

 around the intestine to the left side of the body, and so on. In 

 a large testis growth may have gone on in all these directions. 



The cells which fill the lumen of the immature testis at this 

 time (spermatogonia) are all of the same appearance. The cyto- 

 plasm is very meagre in amount compared to the size of the 

 nucleus, and the outlines of the cells are somewhat irregular (fig. 

 24) ; in the case of cells lying close together the outlines are often 

 hard to distinguish. The structure of the nucleus is exactly 

 Hke that of the type B cells in the germinal layer, from which 

 the spermatogonia have arisen. 



Coincident with the rapid growth and frequent division of the 

 spermatogonia, there occurs a certain increase in the number of 

 the primordial egg cells. Their number in proportion to that of 

 the spermatogonia of course becomes less and less during the 



