The Development of Scyphomedusae. 227 



a breaking through of this cavity to the outside" (p. 5). Before 

 the prostoma closes the embryo is exactly similar in appearance 

 to an invaginate gastrula. Although he could not follow the entire 

 process in Cotylorhiza, he found conditions which led him to 

 believe that here also an ingresson of cells was the process leading 

 to gastrulation. This view was opposed to Claus and in 1891 

 this investigator gave the results of further observations upon 

 Cotylorhiza, Aurelia and Chrysaora. In Cotylorhiza he found 

 no ingression, but a true invagination. Aurelia showed some in- 

 gression of cells which he believed did not take part in entoderm 

 formation; rather an almost typical invagination was the process 

 as he had found earlier (1883). In Chrysaora he confirmed his 

 earlier results. 



As a result of his own work, and from a summary of that of 

 others, Hamann (1890) held to Goette's views that polar ingres- 

 sion was the more common process of entoderm formation in 

 Scyphomedusse. His own observations upon Aurelia aurita, 

 Chrysaora and Cyanea capillata led him to this conclusion, and 

 from the papers of others he believed that a real invagination oc- 

 curred only in Nausithoe and Pelagia, while at least ten other 

 species of Scyphomedusse showed a polar ingression. McMurrich's 

 (1891) observations upon Cyanea artica seemed to confirm this, 

 though he found the ingression to be multipolar. Although the 

 end result was a structure like an invaginate gastrula, invagination 

 had taken no part in the process, according to McMurrich. 



Once more new evidence of invagination was brought forward 

 by Smith (1891) working upon Aurelia flavidula. He found, in- 

 deed, that there was an ingression of cells, which might begin long 

 before invagination started. This ingression resembled that found 

 by Goette in A. aurita, but it was not of constant occurrence; 

 indeed the majority of Smith's preparations did not show any in- 

 gression, though invagination occurred in all. He found further 

 that only three or four cells took part in the ingression, the nuclei 

 of these always broke up into small particles and later the cells 

 degenerated as Claus also found. Sometimes they persisted till 

 gastrulation was completed, but did not take part in the entoderm 

 formation, some of them were even forced through the entoderm 



