156 



ORGANS AND ORGAN SYSTEMS 



sperm mother cells derived from the testes. Strictly speaking, 

 there is but one sac, for the cavities of the seminal vesicles are 

 all in open communication, the walls of the sac being drawn out 

 in three pairs of lobes. The testes are small and difficult to find 

 in the mature worm, for the dorsal wall of the vesicle sac must 

 first be removed. One pair are situated on the posterior side 

 of the anterior wall of the loth somite, and another pair are 

 in the corresponding position on the anterior wall of the nth 



v/// 



FIG. 63. Diagram of the reproductive system of the earthworm showing the 

 central chamber of the seminal vesicles and the internal position of the testes. 



somite. When the sperm mother cells have reached a certain 

 stage of development, they drop off the testis and continue their 

 development as free cells in the cavities of the vesicles (Figs. 

 63 and 64). 



The primordial germ cells which give rise to the spermatozoa are formed 

 in the testes. Here the nuclei divide without cell divisions, until multi- 

 nucleated protoplasmic masses are formed, which break loose from the 

 testes and continue their development in the seminal vesicles (Fig. 64). 



