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 oocyte, 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 ('9P, 

 '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 



