224 Chas. W. and G. T. Hargitt. 



HiSTOEICAL 



It was found by Kowalevsky (1873) that Cassiopea borbonica 

 and Aurelia aurita went through regular cleavage, and a blastula 

 with a relatively small cavity was formed. The entoderm was 

 formed through a small invagination of the blastula wall, and 

 was soon entirely separated from the ectoderm, a relatively larger 

 cavity remaining between the two layers. Glaus (1878) stated 

 that Ghrysaora passed its entire embryonic development within 

 the ovary, emerging as a planula. The egg cells originated in the 

 germinal epithelium and were covered by a follicle developed from 

 this layer. The unequal cleavage, he believed, began while the 

 eggs were still small and continued during the growing period, 

 resulting in a blastula with shorter cells at one pole. From this 

 region an ingrowth of cells took place and the entoderm was 

 formed from these, the cleavage cavity being obliterated and the 

 blastopore completely closed. 



Haeckel (1881) studying Aurelia aurita, agreed with Glaus 

 (Ghrysaora) that the first cleavage was not quite equal, and he 

 referred to a differentiation of the smaller (animal) and larger 

 (vegetal) blastomere, a difference believed to be retained for a 

 considerable time. This inequality disappeared in later cleavage 

 so that the blastula was composed of a large number of equal cells, 

 enclosing a large cavity. The gastrula was formed by an invagi- 

 nation of one side of the blastula, the blastopore closing, but later 

 the larval mouth ('nachmund') breaking through in the same place. 

 As variations from the above processes he found that the cleavage 

 might be quite unequal; that the invagination might be incomplete, 

 the archenteron limited to about one third of the cleavage cavity, 

 the remainder being filled with a jelly-like substance. When the 

 gastrula was formed thus it usually transformed directly into an 

 ephyra, skipping the planula, scyphostoma and strobila stages; 

 or the blastopore of the completely invaginated gastrula remained 

 open and the embryo settled down as an actinula with several 

 tentacles, thus omitting the free-swimming planula stage. He also 

 agreed with some earlier authors that the planula might bud or 

 divide into several planulse each of which formed a scyphostoma. 



