The Development of Scyphomedusse. 231 



tions in the gastrulation of Aurelia aurita; and the later work of 

 Hyde upon three species of Scyphomedusse showed other varia- 

 tions in the gastrulation even within a single species (A. flavidula). 

 Hence there seems no reason further to insist upon the activity 

 of any one, or any single process in entoderm formation. Nor 

 does there appear to be any occasion to hold any longer to the 

 view that differences in the gastrulation process have any neces- 

 sary significance in phylogeny. Conklin has given expression to 

 this view when he says (p. 163), " . . the form of gastrulation 

 is of no fundamental or general significance, but that it depends 

 upon individual or environmental conditions." 



The observations described in the following sections are not 

 intended to add anything essentially new to the controversy 

 regarding gastrulation, but to record the facts brought out 

 in a study of Cyanea arctica and Aurelia flavidula, the develop- 

 ment of the former species never having been fully worked out. 

 It may be said, however, that the results of this study and a care- 

 ful comparison of the earlier papers has led to the conclusion that 

 invagination is probably a more general and dominant method of 

 entoderm formation in the Scyphomedusse than was thought by 

 Goette, Hamann and others. 



Cyanea Arctica 



The development of Cyanea up to the formation of the planula 

 takes place within the folds of the mouth lobes as stated in an 

 earlier paper (1902 b) . Cleavage stages are passed through rather 

 rapidly so that only the later stages are found in most medusse. 

 Specimens obtained in early spring 1908, however, gave material 

 which makes possible the study of the early development, though 

 not the oogenesis and fertilization. In no case were there found in 

 the mouth lobes more than an occasional egg which had not 

 already begun to segment ; hence the formation of polar bodies and 

 fertilization must occur either within the gastric cavity, or before 

 the eggs leave the ovary. The conditions in Aurelia, which are de- 

 scribed later, suggest the probability that the formation of the 

 polar bodies occurs at about the time of the escape of the eggs from 



JOURNAL OF MORPHOLOGY — VOL. 21, NO. 2. 



