870 JSTaohide Yatsu. 



both ill number and in longtli. Soon afterwards the middle-piece 

 is found thrown off into the cytoplasm, where it eventually fades 

 (PL II, Figs. 35 and 36). Now the ccntriole surrounded by astral 

 rays lies free in the egg close by the spcrni-nucleus. How the cen- 

 triole escapes from the middle-piece I do not know. It may be due 

 to the movement of the sperm-nucleus and aster, the middle-piece 

 being left behind. As a matter of fact I have a few cases in which 

 the middle-piece and the tail together are found detached from the 

 sperm-nucleus at the stage represented in Fig. 29 (PI. II). (Wilson, 

 '97; Foot and Strobell, '03, their Fig. 9.) 



When the centriole escapes from the middle-piece, the centroplasm 

 has not made its api^earance, the rays reaching the centriole. Soon 

 after, the central ends of the rays become obscure and the centro- 

 plasm is formed. In it the centriole divides into two. The divi- 

 sion i^lane has no definite relation to the egg radius (cf. PL II, 

 Figs. 36 and 37). From this stage on two different types may be 

 distinguished in the formation of two daughter asters: (A) in 

 many eggs the centrosome disappears immediately afttu" this. The two 

 naked centrioles are found surrounded by new short rays. As they 

 are separated farther and farther from each other, the rays (fertili- 

 zation rays) grow to a considerable length (PL II, Figs. 37 and 

 38). A spindle is formed by a secondary connection of the rays 

 between the two asters. (B) In a few cases I find the sperm-aster 

 Avith much centroplasm (PL II, Fig. 39). The rays are coarse and 

 not so numerous as in type A. The centroplasm increases in quan- 

 tity and eventually becomes spongy (PL II, Fig. 40). In it the 

 two daughter centrioles acquire new r,ay-systems and the old rays 

 gradually fade away. These two types differ from each other simply 

 in the period of aster division owing to different amounts of cen- 

 troplasm ; in type A the aster division is completed very early, while 

 in type B it has not yet finished even as late as a stage shown 

 in Fig. 40 (PL II). It should be remarked that type A resem- 

 bles what was observed by Coe on C. marginatus, differing only 

 in one point; that is, in the I^eapolitan species a large number of 

 old rays directly pass into the daughter systems ('99, p. 446). Type 

 A resembles the formation of the asters for the second maturation 



