244 FIELD. [Vol. XI. 



The general history is in its first and simplest aspects a 

 series of repeated mitotic divisions. The number of these 

 divisions in the case of echinoderms is but two. By the first 

 the spermatogone gives rise to two spermatocytes ; by the 

 second each of these spermatocytes forms two spermatids. 

 No later mitoses occur, and each spermatid then by a series of 

 changes of the constituent parts of the cell become transformed 

 into a spermatozoon. The general facts to be noted are, first, 

 that the number of divisions is but two, instead of a large or 

 even an indefinite number as is the case in certain animals, 

 and as was believed to be the case in echinoderms ; and sec- 

 ondly, that both divisions are by mitosis. The spermatogones 

 measure ii-13/x in diameter; the spermatocytes S-io/l* ; the 

 spermatids 5-7/*, as found in teased preparations. Sections of 

 the alveoli of the testis (Fig. 12) show zones pretty sharply 

 characterized by the various stages in the history of the devel- 

 opment of the spermatozoa from the spermatogones. Thus 

 the external series of cells, i.e., those next to the germinal 

 epithelium are spermatogones with the nucleus in a resting 

 condition, having large nucleoli. Next to this is a zone where 

 the nuclei are in the various stages of mitosis ; next a zone 

 made up of spermatocytes ; then come the spermatids, and 

 towards the center of the lumen the spermatids in process of 

 change into spermatozoa ; then the immature, and nearer 

 the center the ripe spermatozoa. In certain cases of starfish 

 studied in January, nucleoli were noted in the spermatocytes : 

 this is probably due to the fact that spermatogenesis was not 

 going on actively at that season. Ordinarily, however, there 

 seemed to be no resting periods intervening between the 

 mitoses. 



In the echinoderm phylum there exists a very constant and 

 considerable difference in the shape and size of the spermato- 

 zoon of each class, and even within the class a slight differ- 

 ence in the shape and size of the spermatozoon of the different 

 species. (Compare Figs, i, 2, 3, 4 and 5.) The Holothurioidea 

 as a group have the largest spermatozoa (Fig. i); next in order 

 of size come the Ophiuroidea (Fig. 4) ; and then the Asteroidea 

 (Fig. 5). In these three classes the head of the spermatozoon 



