252 FIELD. [Vol. XI. 



strong acetic acid excepted (Fig. 23); even when the nucleus 

 swells and bursts, the centrosome as well as the mitosome 

 remains uninjured. 



In the living spermatozoon in sea-water the centrosome can be 

 seen as a brighter, more refringent spot near the anterior end, 

 in the case of the Holothurioidea, Asteroidea, and Ophiuroidea 

 examined (Figs, i, 4, 5). This fact was noticed by Cuenot (4). 

 It was in 1883 seen by Jensen (13), but he interpreted it as 

 merely a depression at the anterior end of the nucleus caused 

 by the shrinkage due to expulsion of the achromatin from the 

 nucleus. 



As to the further history of the centrosome : the sperm cen- 

 trosome was seen and figured by O. and R. Hertwig, Bovieri, 

 and Fol, in the fertilized echinoderm ^^g. But, so far as I am 

 aware, no one has previously worked out the history of this 

 centrosome. The Hertwig brothers believed that it came from 

 the middle piece, since it showed the same micro-chemical 

 reactions. Fol believed that the anterior portion of the 

 nucleus became fragmented off. Pictet pointed out the erro- 

 neousness of Hertwigs' and inclined to Fol's view. But, as 

 shown above, the sperm centrosome is the centrosome of the 

 spermatid, and of the previous cell-generations, and it continues 

 in company with the nucleus of the spermatozoon until the 

 completion of the process of fertilization. As soon as it has 

 passed the denser outer layer of the cytoplasm of the Q.gg it 

 draws farther away from the nucleus, and the characteristic 

 radiations appear in the cytoplasm. It no longer directly pre- 

 cedes the nucleus, but comes to lie at one side (Fig. 58). 



The Mitosome. — The very small granules, darkly staining 

 with dahlia, visible in the nucleus before the disappearance of 

 the nuclear membrane (Fig. 29), and which later become the 

 fibres of the mitotic spindle (Fig. 31), have been already re- 

 ferred to (p. 246) ; as well also the fact that after the division of 

 the spermatocyte into the spermatids, these granules, which were 

 the mitotic spindle of the spermatogone and spermatocyte, are 

 no longer included within the nuclear membrane, but are scat- 

 tered through the cytoplasm (Figs. 32, 33, 34). With the 

 beginning of those changes which mark the transformation of 



