448 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
forms while studying the living eggs in a small preparation dish in 
seawater with a two-thirds objective. As the eggs present a number 
of different forms when subjected to the same external conditions, 
it seems that the cause of these differences must be sought in the nature 
of the egg itself rather than in any surrounding influences. 
The later cleavages follow at intervals of about the same duration 
as in the preceding stages. The irregularities of arrangement of the 
blastomeres increase as the cells become more numerous. On account 
of the smallness of the blastomeres and the extreme opacity of the 
egg, it becomes impossible to follow the segmentation in detail any 
farther. Figures 27-30 of plate 33 show a few of the later stages of 
comparatively very regular forms. Figure 29 (pi. 33) represents an 
egg in which the blastomeres are arranged in two main groups held 
together by a narrow isthmus of only one cell in thickness. Some 
eggs were separated into three or four thickened clusters that were 
joined together by smaller masses o;f connecting cells. In others 
there were smaller groups of blastomeres projecting out from the 
general mass of cells, thus giving the whole somewhat of an amoeboid 
appearance. The term amoeba-like seems most clearly to represent 
the shape which some of these late segmentation stages assume, for 
if a simple outline of these remarkable and grotesque forms is drawn, 
it has a general resemblance to an amoeba with thick, blunt pseudo¬ 
podia. Whether these irregularities in the shape of the egg during 
late segmentation, and the tendency of the cells to arrange themselves 
into more or less distinct lobes are due to an amoeboid property of the 
cytoplasm of the egg, or to a tendency to multiply by division during 
cleavage, as was suggested by Metschnikoff for Oceania armata, there 
is not sufficient evidence to decide. It may be possible that both 
these factors act in determining the shape of the segmenting mass of 
cells. Doubtless the membraneless character of the egg plays a part 
in these phenomena. 
Planula. 
When segmentation is complete a solid embryo is formed which 
may at first be called a morula. Small spaces occur sometimes between 
the blastomeres during the different cleavage stages, but they are 
sooner or later obliterated by the crowding; together of the cells. A 
central cleavage cavity which is later transformed into a blastococle 
