448 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 



forms while studying the living eggs in a .small preparation dish in 

 scawater 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 of 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 blastocoele 



