No. 2.] THE ECHINODERM SPERMATOZOON. 247 



The Nucleus. — The nuclei of the spermatogones in the outer 

 zone, as seen in a section of the testis, usually alone of all the 

 cells show nucleoli. The nucleus is large relatively to the 

 amount of cytoplasm. It very soon begins the process by 

 which it will ultimately give rise to the spermatozoa. It 

 divides by mitosis and forms the nuclei of two spermatocytes. 

 The number of chromosomes into which the nuclein of the 

 spermatogone collects seems to be 28-36 : in the spermatocyte 

 16-18. The attempt to count with exactness so small and so 

 numerous bodies, so closely crowded together, is well nigh 

 fruitless. The dividing nucleus plainly is seen to be made up 

 of several substances, e.g., nuclein, staining deeply with methyl 

 green ; the caryoplasma staining lightly ; and another sub- 

 stance which has the appearance of minute granules; these 

 evidently form the mitotic spindle. A centrosome is also 

 present. These granules and the centrosome take a violet 

 color with dahlia (Fig. 29). 



The nucleus of each spermatocyte has the same constituent 

 parts as that of the spermatogone. It divides by mitosis and 

 forms the nuclei of two spermatids. The nucleus of each 

 spermatid contains eight or nine chromosomes, and caryo- 

 plasma. Within the nucleus there seems to be no signs of 

 the granules which formed the nuclear spindle ; but these 

 granules and the centrosome are now very distinctly seen to 

 be in the cytoplasm (Figs. 32, 33). It should be noted that 

 each of these mitoses are "reducing divisions." 



With the spermatid begin those changes in the shape and 

 constitution of the nucleus which are connected with the speci- 

 alized form of the spermatozoon, (i) The change in shape. In 

 the case of the Holothurioidea, Asteroidea, and Ophiuroidea 

 the change is insignificant, usually only a slight flattening in 

 the antero-posterior direction. But with the Crinoids and with 

 the Echinoidea the nucleus gradually changes from spherical to 

 conical. (2) There seems to be a change in the constitution 

 of the nucleus, as remarked by Pictet (18). I can confirm his 

 observation. The chromosomes (usually nine, sometimes eight) 

 can be demonstrated in the spermatids and in certain of the 

 spermatozoa, probably the immature ones. But in others the 



