90 MARINE BIOLOGICAL LABORATORY. 



dusae delamination may produce a hollow diploblastic 

 embryo without the intervention of a solid condition. 

 The formation of a solid morula during segmentation 

 which occurs in such forms as Hydractinia, Clava, etc., 

 may readily be regarded as a precocious immigration, 

 finding its counterpart among the Sponges in Chalinula. 

 (Kellar). 



In the Scyphomedusae some forms such as Pelagia^ 

 Cyajiea, and Chiysaora have long been considered to 

 have an invaginate gastrula. Regarding Pclagia there 

 is no doubt that this is really the case, and Metschnikoff 

 has described the occurrence of invasfination in Natisi- 

 tJioe. As regards the other forms, however, the observa- 

 tions are not sufficiently complete to render the occur- 

 rence of this process certain. Sections are absolutely 

 necessary for a correct determination of the processes 

 which occur, and Goette's observations on Aiirelia and 

 Cotylorhiza show that a structure exactly resembling 

 an invaginate gastrula may be produced from a solid 

 planula formed by migration. My own observations 

 on Cyanea arctica demonstrate the occurrence in its 

 development of a solid planula formed apparently by 

 immigration, and it seems probable from Claus' figures 

 that such is also the case in Chrysaoi-a. 



Among the Anthozoa it is very doubtful if invagina- 

 tion ever occurs. In the majority of forms investigated 

 delamination is the process by which the inner cells 

 are formed, but invagination has been stated by Kowa- 

 lewsky to occur in Ceriaitt/ius mevtbranaceus and Acthzia, 

 sp. (.'*) In Metridium marginatum I have found what 

 renders doubtful the statements of that author, who 

 studied optical sections only. A hollow blastula results 



