288 



BULLETIN : MUSEUM OF COMPAKATIVE ZOULOGY. 



VI. 



PAGE 



i. The metamorphosis of the 



spermatid . . . 375 



a. The eentrosome . . 375 



b. The archoplasmic 



structures .... 376 



c. The axial filament . 377 

 5. Fertilization 378 



Summary 379 



Somatic mitosis 379 



Spermatogonia 379 



Primary spermatocytes . . 380 



PAGE 



Secondary spermatocytes . 380 

 Metamorphosis of the sper- 

 matids 381 



Oogenesis 382 



Oocytes 382 



Growth period 382 



Fertilization 382 



Cleavage 383 



Bibliography 384 



Explanation of plates 



I. Introduction. 



In the numerous cytologic investigations of the last ten years the 

 sponges and coelenterates have been so neglected that we know to-day 

 very little of the finer structure of their nuclei, either in the " resting " 

 condition or in mitosis. This is the more remarkable when we recall what 

 valuable objects these forms have been in other lines of research, and the 

 interest a knowledge of their nuclear structures must possess on account 

 of their simple organization. In connection with such subjects as sper- 

 matogenesis, the individuality of the chromosomes, the disposition of 

 the chromatin in the resting nucleus, and the origin and fate of nucleoli, 

 one might not unreasonably hope to gain an insight into ancestral con- 

 ditions of importance in the phylogenetic study of the nucleus. These 

 considerations have seemed sufficient reason for undertaking the present 

 research. 



The research was originally planned to cover the spermatogenesis of 

 one member of each of the four great groups of coelenterates ; but at 

 the outset I was confronted by these obstacles : in most cases the male 

 germ cells are very small, the chromatin in the critical stages exceed- 

 ingly crowded, and the satisfactory cytologic fixation of the tissues 

 attended with difficulty. Of the various forms examined the hydrome- 

 dusa Gonionemus murbachii proved most satisfactory in all these re- 

 spects; it therefore seemed wise to limit the present study to this one 

 species, and to endeavor to follow the entire nuclear cycle from adult 

 somatic cell through the germ cells of both sexes and through fertiliza- 

 tion to the cleavage nucleus, and so back to the adult tissue, in the hope 

 of thus laying a firmer foundation for later comparative studies than 

 could otherwise be done. At present I have studied the ordinary mi- 

 tosis of somatic cells, the cleavage spindles, the entire course of sperma- 

 togenesis, the early nuclear development of the oocyte, and the nuclear 



