No. 2.] THE ECHINODERM SPERMATOZOON. 249 



essential part, for in this case most of the male caryoplasma 

 passes out into the cytoplasm of the male cell, and the caryo- 

 plasma is replaced by liquid from the cytoplasm of a female cell. 



The nucleus varies considerably in size and shape in the 

 different classes, as a comparison of the table, p. 246 and figures 

 (1-5) will show. In Antedon and the Echinoidea it is compar- 

 atively small ; while larger in the Asteroidea it is largest in the 

 Holothurioidea and Ophiuroidea. In the anterior surface, usu- 

 ally at the very apex, is a depression into which the centrosome 

 fits. In the Echinoidea, however, it is often not at the very tip 

 of the sharp-pointed conical nucleus, but on the side, usually, 

 however, very near the anterior end (Figs. 24, 25, 26). 



Centrosome. — I have never seen a centrosome in the sper- 

 matogones in the outer zone, i.e., next to the germinal epithe- 

 lium. I first succeeded in finding it in the dividing sperma- 

 togones. At first it seemed to be within the nuclear membrane. 

 But observation on this point is very difficult, and I am by no 

 means positive in regard to the manner or place of first appear- 

 ance. I have never seen in the spermatogone the actual divi- 

 sion of the centrosomes, but it is probable that it occurs 

 preparatory to the mitotic division. With the disappearance 

 of the nuclear membrane the centrosomes are seen to have the 

 usual position at the poles of the nuclear spindle. This spin- 

 dle seems to be formed of those violet-staining granules which 

 are present in the nucleus before the disappearance of the 

 nuclear membrane (Figs. 29, 30, and 31). As shown in Figs. 

 30 and 31, these granules remain in close proximity to the 

 chromosomes, the cytoplasm being entirely free from them, 

 previously to the final division, which gives rise to those cells, 

 the spermatids, which will become the spermatozoa. After 

 the division of the nucleus of the spermatocyte into the two 

 spermatid nuclei, and before the division of the cytoplasm has 

 taken place, those granules which composed the spindle fibres 

 are seen scattered through the cytoplasm (Figs. 32, 33) ; the 

 nuclei themselves, rounded up and with a nuclear membrane, 

 are free from the granules. In the cytoplasm also, close beside 

 each nucleus remains one of the centrosomes, which took part 

 in the division of the cell (Figs. 32, 33, 34, 35). 



