450 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
swimming movements are lost, the plannla is capable of gliding along 
the bottom of the dish for some time. Finally motion ceases alto- 
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get her 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 plannla 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. 
V 
Brooks describes and figures an ectodermal invagination at the 
posterior end of the plannla. 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 
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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 seawater. Then with a 
