o 



36 bulletin: museum of comparative zoology. 



oocyte. From this point on to the close of the second maturation 

 division a gap, due to lack of material, still exists in my observations. 

 Although the portion of the oogenetic process here described is less im- 

 portant in counection with the questions of reduction than are the 

 later stages, yet, it does cover certain phases of importance from a more 

 general cytologic standpoint. It is of theoretic interest to establish 

 conclusively in this primitive class, whether the chromosomes retain 

 their individuality from the close of the last oogonial mitosis throughout 

 the growth period of the oocyte, as Hacker ('92 b ) found to be the case 

 in some copepods, and Etickert ('92) in selachians ; or whether, as is no 

 doubt the case in the majority of Metazoa, the nuclear contents enter 

 during this period into a resting condition with diffuse chromatin. 



I have not studied the younger stages in the formation of the gonad 

 in Gonionemus, and so have not been able to attack the question of the 

 origin of the female germ cells in that genus, but from conditions easily 

 traced in Olindias, a closely allied form, I believe that they are of ecto- 

 dermic origin and originate in the region of the gonad, not migrating 

 from elsewhere, as is so commonly the case among the Anthomedusae 

 and Leptomedusae. In all the many sections of radial canals examined, 

 I have never found any evidence of the existence of any such migratory 

 cells in Gonionemus. 



The ovary (Fig. 1) is, in its general features, similar to the spermary, 

 presenting the appearance well known and often described among those 

 hydromedusae whose sexual products are borne along the radial canals. 

 A section invariably shows a number of nearly ripe eggs, others smaller, 

 and, packed between these, often basally, small groups of oogonia and 

 young oocytes. Since this structure is typical, it is unnecessary to de- 

 scribe it more fully here, and we may proceed at once to consider 



1. Tlie Oogonia. 



In sections of the gonad these cells (Fig. 1, o'go) are seen in small 

 groups of from five to ten, in the spaces between the ripening oocytes, 

 frequently in contact with the mesogloea, but occasionally in a more 

 peripheral position ; indeed, the only part of the gonad where they do 

 not regularly occur is at its surface, which is covered by a layer of ecto- 

 derm cells easily distinguished by their exceedingly small nuclei. The 

 oogonia are not veiy numerous, and appear to divide but rarely, or at 

 least at long intervals ; so that it has proved much more difficult to 

 trace the stages in mitosis than is the case with the spermatogonia. It 



