HABITS, ETC., OF CEREBRATULUS LACTEUS. 127 



found that they were not developed in preformed sacs, but 

 that they themselves gave rise to the latter, or rather 

 furnished the occasion for their existence. A few large cells 

 appeared in the body parenchyma, which were soon gathered 

 into bunches, around which was developed a thin membrane, 

 thus forming a testis. But in Cerebratulus the spermatozoa 

 as well as the eggs are developed from the epithelium lining 

 the genital sacs. The cells of this epithelium increase rapidly 

 by division, and the original nuclei become larger, as in the 

 development of the ova. Through the consequent crowding 

 of the cells some of them are pushed out into the central 

 cavity, where they become sperm mother-cells and form 

 spermatozoa by segmentation (fig. 66) . The transition from 

 the sperm mother-cell, or spermatid, into the spermatozoon 

 is apparently not direct, there being intermediate stages 

 (fig. 68). ■ 



But the chromatic substance of the nucleus of the sper- 

 matid gradually separates itself as the head of the sperm, 

 while the body draws itself out into a long flagellum (cf. 

 figs. 67 and 68). The general type of development is thus 

 very similar to that given by Lee (loc. cit.), the essential 

 difference being that in Cerebratulus the testis is preformed 

 and the sperms are developed from its epithelium. 



When fully ripe the sperms are very large (0*05 mm. long) 

 and of a peculiar shape (fig. 59 ; compare also fig. 42, in which 

 a sperm and the polar bodies are drawn to the same scale). 



The head is long and sickle-shaped, tapering to a very 

 fine point at the anterior end, and usually somewhat curved 

 into a crescent. The middle piece is nearly spherical, and 

 caps the posterior end of the head like the top of a clothes- 

 pin. 



The tail or flagellum is very long, many times the length 

 of the head and middle piece, and is undivided. 



These sperms have a slow movement, but it is seemingly 

 very strong, for they experience no difiiculty in getting 

 through the two egg membranes in order to reach the egg 

 itself* 



I 



