226 Chas. W. and G. T. Hargitt. 



cells of the blastula were so nearly equivalent in size that no polar 

 differentiation was possible. The blastula which (unlike Aurelia 

 aurita) had a very large cavity, took on a somewhat elongate 

 form, the cells of one side became thicker and at this point an 

 invagination occurred. Even during the invagination the cells 

 began to be differentiated from the ectoderm cells and when the 

 blastopore had closed there was a typical planula with two layers 

 differentiated. Pelagia noctiluca differed mainly in the wider ex- 

 tent of the invagination and in the absence of an early differ- 

 entiation of the entoderm cells. Also during invagination the 

 ectoderm grew so that the two layers came to be further apart 

 than at first. The invagination in Pelagia was confirmed much 

 later by Goette (1893). Metschnikoff thus confirmed the earlier 

 workers on the formation of the entoderm by invagination, and he 

 believed this process to be a wide-spread one in the Acraspedota 

 [Scyphomedusse] . 



This general belief in the wide extent of invagination among the 

 Scyphomedusse was vigorously attacked by Goette (1887), who 

 claimed that Aurelia at least, and probably in other Scyphomedu- 

 sse, gastrulation had been erroneously described. This was due, 

 in his opinion, partly to the fact that sections has not been used 

 in the study of the gastrulation. Furthermore since a coeloblastula 

 was the starting point of the gastrulation and a coelogastrula the 

 end result, it seemed to be necessary and was natural to assume 

 that in the stages between, an invagination had taken place. But 

 ''such an invagination does not occur in Aurelia" either as des- 

 cribed by Haeckel or by Glaus. A sterrogastrula is formed, " . . 

 and in Aurelia repeating exactly this process, the gastrulation 

 occurs through a cell-inwandering." The cells which took part 

 in the ingression came from the region of the blastula made up of 

 shorter cells, and were either entire cells, or portions of cells set 

 free by a process of delamination. The cleavage cavity thus 

 became filled with a mass of cells, the entoderm, which by splitting 

 apart assumed a position in a single layer. The ectoderm and 

 entoderm then fused at one point and a prostoma was formed: 

 "archenteron and prostoma therefore arise not through invagi- 

 nation, but through the hollowing out of a massive entoderm and 



