190 bulletin: museum of comparative zoology. 



the cytoplasmic division completed before the next nuclear division 

 started. This delay in the cleavage of the egg is so extreme in some 

 instances that cytoplasmic division does not begin till a number of 

 nuclei are present. Similar conditions are common in other coelenter- 

 ates, and have already been indicated by other observers. 



The result of the first and second cleavages may be respectively two 

 and four equal blastomeres, as is foreshadowed in Figure 58. The 

 eggs which I have seen at the end of the first cleavage, in cases where 

 the division of the egg followed immediately on that of the nucleus, 

 showed approximately equal parts; but the second cleavage may 

 result in the formation of very unecjual parts, and from this time on 

 inequality is the rule. I have no doubt that the first division may also 

 sometimes result in unequal blastomeres. Figure 59 illustrates the 

 inequality in the size of the blastomeres resulting from the second 

 cleavage. 



b. Later Cleavage. — Usually the cleavage planes for a considerable 

 time correspond to meridional divisions in eggs that segment more 

 regularly. An equatorial division rarely occurred before the 12-cell 

 stage, and usually it was much later in making its appearance. Sec- 

 tions from eggs with 20-30 nuclei each are shown in Figures 60-62; 

 the meridional furrows, the delay of the equatorial cleavage and the 

 polarity of the egg, all being well shown. The elongated shape of 

 these eggs is probably due to external causes, the development of 

 several eggs in one gonophore having led to a compression which has 

 influenced the direction of growth, and may have helped to determine 

 the position of the planes of cleavage. 



Traces of segmentation cavities are seen in Figures 60-62, and such 

 cavities, I believe, usually occur. In this I must differ from Allen 

 (:00), who did not in any instance find a cleavage cavity in Tubularia. 

 It is clear from these sections that the cleavage cavity is not a typical 

 one, such as we know in eggs of echinoderms, for instance. Several 

 separate spaces occur between adjacent cells, beginning as early as the 

 8-cell stage. It is doubtful whether these spaces unite to form a 

 single cavity in all cases, perhaps they do not even in the majority of 

 cases; there is no question, however, that a single cavity does arise in 

 some instances. For example. Figure 63 (Plate 8), a section of an 

 egg with 40-50 cells, shows a rather irregular, but single segmenta- 

 tion cavity, which is for the most part surrounded by a single layer 

 of cells. This stage, I believe, corresponds to the typical blastula of 

 other animals, from which it differs chiefly in its irregularity of form. 

 That this stage, which marks the end of segmentation, does not always 



