GERM CELLS OF COELENTERATES 595 



ated eggs that one would look for fertilization and cleavage; 

 the difficulty of securing these eggs has resulted in leaving those 

 phases of development out of consideration. Whether there is 

 any periodicity in the time of liberation of eggs was also left 

 undetermined. 



The eggs are relatively rather small as compared with those 

 of many Coelenterates, but the nucleus is relatively much larger 

 than in any other coelenterate egg which has come under my 

 observation. There is a sharpness and clear cut definition in 

 the nucleus and its elements which renders this form peculiarly 

 favorable for study. The chromatin masses are well defined 

 and the chromosomes are very large and few in number, which 

 makes their study easy. All in all, the egg of Aglantha is the 

 most favorable of all the Hydrozoa which I have studied. Many 

 stages of the spermatogenesis have been observed, but as these 

 cells are small (though the nucleus is large) it is difficult to deter- 

 mine all phases of the process. Because of the incompleteness 

 of the observations on the development of the male germ cells 

 I have not included them here. 



ORIGIN OF GERM CELLS 



The material used for this study was abundant and covered 

 the entire period of oogenesis except the very earliest stages. 

 In the earliest medusae obtained the gonads were tiny spots 

 scarcely noticeable without magnification; to secure earlier 

 stages in oogenesis very immature specimens would be essential 

 and these probably would be found before the time when medusae 

 are abundant. In the youngest specimens obtained the tiny 

 gonads had already passed the stage of primordial germ cells 

 and a considerable number of oogonia or of spermatogonia were 

 present. The method of formation of germ cells was therefore 

 not determined, but their place of formation was quite clear. 



The gonads are made up of an entodermal epithelium lining 

 the cavity, a mass of germ cells and a thin ectodermal tissue 

 covering the outside. In young gonads it is clear that they 

 arise by an evagination of the radial canal to form the cavity 

 of the gonad, and a proliferation of the ectoderm cells to form 



