200 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
of fragmentation. Moreover, I have been able to establish a direct 
descent of the cleavage nuclei of the blastomeres from the germinative 
vesicle of the growing odcyte, through the maturation spindles, the 
germ nuclei, and the first segmentation nucleus, each division being by 
mitosis. No step is lacking, and in ‘Tubularia I have traced this line 
of descent into the nuclei of both ectoderm and entoderm of the planula, 
and have found each nucleus arising by a typical mitosis. 
Brauer (’91) distinguished two methods of segmentation in Tubu- 
laria mesembryanthemum: (1) a regular segmentation of the egg 
following nuclear division, and (2) a nuclear proliferation not imme- 
diately followed by the differentiation of cells, this taking place only 
at a later period. Hargitt (:04*) showed that while such differences 
exist, there is no sharp distinction between them. Allen (:00) found 
essentially the same conditions in ‘Tubularia crocea, and my observa- 
tions are confirmatory, though I often found in the cleavage a regu- 
larity which she never observed. As has often been found in Hydro- 
medusae, the cleavage of the egg is delayed, and any nuclear division 
may be under way, or even finished, before the egg has completed the 
segmentation corresponding to the immediately preceding nuclear 
division. 
In the late development of the egg a solid mass of cells is character- 
istic of nearly all Hydromedusae. ‘This has been interpreted in two 
ways: Conn (’82), Allen (:00), Harm (:02), Hargitt (:04°, :04°), 
Stschelkanowzew (:06), Brooks and Rittenhouse (:07) think this 
represents the end of segmentation, no cleavage cavity having been 
formed, i. e., the solid mass of cells is a true morula. On the other 
hand, Claus (’82), Hamann (’83), Merejkowsky (’83), Brauer (791°, 
’91b), Gerd (’92), Hacker (’92), Bunting (’94), Morganstern (:01), 
Wulfert (:02), Tannreuther (:08), and others have found a cleavage 
cavity during the course of segmentation, and consider that a blastula 
is formed, which represents the end of segmentation. ‘The solid mass 
of cells, according to this view, is not a true morula, but in part a 
result of the formation of the germ layers. 
The conditions found in Tubularia crocea agree with the second 
view, viz: that there is a blastula, but not a true morula. That a 
blastula is sometimes formed in ‘Tubularia crocea admits of no ques- 
tion, as some of my figures show. However, there is as little question 
that the segmentation cavity may be reduced to more or less separated 
spaces between the cells, and possibly may sometimes be entirely 
lacking. ‘This reduction of the segmentation cavity may be due, in 
part, to the fact that the development takes place within a closed 
Sate Fa ae +4 
