158 ORGANS AND ORGAN SYSTEMS 



spermatozoon, which remains attached to the parent protoplasmic mass 

 (blastophore) ; the centrosome forms the middle piece, while the cyto- 

 plasmic 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 op- 

 posite each testis there is a large convoluted, ciliated opening 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 rela- 

 tively 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 

 openings of the sperm receptacles. When the girdle passes off 

 the anterior end, it closes at the front end and afterward at the 

 posterior end. The girdle thus forms a cocoon, which hardens 

 later into a chitinous spindle-shaped vessel containing repro- 

 ductive cells and albuminoid food material (Fig. 65). The eggs 

 are fertilized in the coccoon by the spermatozoa, and develop- 

 ment begins at once, continuing under the protection of the 

 cocoon. 



Cleavageof the egg is regular up to the i6-cell stage, with four 

 vegetative cells at the lower, and smaller animal cells at the 

 upper pole. The lower cells invaginate and form a typical two- 



