450 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 



swimming movements are lost, the planula is capable of gliding along 

 the bottom of the dish for some time. Finally motion ceases alto- 

 gether and the larva loses its cilia and is ready for attachment. This 

 stage of development is reached under favorable conditions from about 

 forty-eight to fifty hours after the eggs have been laid. 



The planula is very opaque, and thus it is impossible to make out 

 anything about its internal structure in studying the living forms. 

 Specimens in various stages of development were preserved and sec- 

 tioned for the study of cellular structure. The description of this 

 structure will be given in connection with the formation of the germ 

 layers. 



Brooks describes and figures an ectodermal invagination at the 

 posterior end of the planula. He says: "In a living planula it is easy 

 to make out the posterior end, an ectodermal invagination, which 

 looks very much like the mouth of an invaginate gastrula, but this 

 resemblance is misleading, for the careful study of a similar structure 

 in the planula of Eutima shows that the invagination has no connec- 

 tion with the digestive cavity, but is an ectodermal gland for the attach- 

 ment of the planula." From my observations I am forced to regard 

 this structure, which he describes, as a variation rather than a normal 

 feature. It seems to be an abnormal occurrence which is found only 

 rarely. Among the many specimens which I studied both in life 

 and from preserved material, such an invagination was met with only 

 on one occasion. Then it was at the anterior end of the planula 

 instead of the posterior. These structures are clearly abnormal 

 features of the developing Turritopsis planula. 



EXPERIMENTAL. 



The very irregular character of the segmenting egg and the loose 

 connection of the blastomeres, and their tendency to separate into 

 more or less definite lobes and protuberances, as has been described 

 in the section on segmentation, suggested the problem: what would 

 be the effect of dividing the eggs during the comparatively early stages 

 of cleavage? With this question in mind a few experiments were 

 tried. The eggs were divided during several stages of segmentation. 

 The best method for separating the cells was found to be by placing 

 them on a clean glass plate moistened with sea water.' Then with a 





