154 ORGANS AND ORGAN SYSTEMS 



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 

 found in the testes. Here the nuclei divide without cell divisions un- 

 til multinucleated protoplasmic masses are formed which break loose 

 from the testes and continue their development in the seminal vesicles 

 (Fig. 64). The nuclei increase by division and take a position at the 

 periphery of the protoplasmic mass where further multiplication follows 

 until there are from 32 to 64 nuclei. Protoplasmic furrows then cut 

 in around each of the peripheral nuclei. Each of these nuclei then 

 begins to elongate and to transform into a spermatozoon. The nu- 

 cleus forms the head of the spermatozoon which remains attached to the 

 parent protoplasmic mass (blastophore) ; the centrosome forms the middle 

 piece while the cytoplasmic tail grows out at the distal end. The 

 bulk of the spermatozoon thus is derived from the nucleus. 



Under the dorsal wall of the median vesicle and directly 

 opposite each testis there is a large convoluted, ciliated open- 

 ing of the sperm duct. These ciliated funnels draw the mature 

 spermatozoa into them and thence they are conducted to the 

 outer opening of the ducts on the i5th somite. The ducts from 

 the ciliated funnels on each side of the worm unite to form a 

 common duct leading to the i5th so that two common sperm 

 ducts, known as the vasa deferentia open on the ventral surface 

 (Fig. 63, sperm duct). 



Female Organs of Reproduction. These are much simpler in 

 .structure than the male organs, consisting of one pair of re- 

 latively large ovaries on the posterior face of the anterior wall of 

 the 13 th somite. The eggs, when mature, drop into a large- 

 mouthed thin-walled funnel-like oviduct which opens on the 

 ventral surface of the i4th somite (Fig. 63). 



J. REPRODUCTION. FERTILIZATION AND DEVELOPMENT. 

 Fertilization of the earthworm eggs takes place after copulation 

 which leaves the sperm receptacles of the worm filled with 

 mature spermatozoa. A tough resistant girdle is formed 

 around the clitellum of each worm and after the worms separate 

 this girdle is worked forward collecting albumen from the glands 

 on the ventral surface, mature eggs as it passes the i4th and 

 mature spermatozoa as it passes the gth and loth somites or 



