192 bulletin: museum of comparative zoology. 



several entoderm cells have already been cut off, and the radial position 

 of some of the spindles clearly shows that this number will be increased 

 by the approaching divisions. Figures 69 and 70 represent more 

 advanced stages in this process, Figure 70 especially showing that the 

 entoderm cells are formed by a "multipolar delamination." The 

 ultimate result of this process is the formation of a solid mass of cells 

 which, in this and other coelenterates, has often been called a morula, 

 and considered to mark the end of segmentation. It seems clear that 

 in Tubularia crocea, where a true blastula occurs, this process should 

 be regarded as a part of the germ-layer formation. The cells com- 

 posing the layers of this solid mass, it is true, do not assume their 

 definitive positions and relations until somewhat later, as the result 

 of further divisions and rearrangement; but that the separation of 

 •entodermal cells has already begun cannot be doubted. During the 

 formation of the germ layers the interzonal filaments of the mitotic 

 figures persist for a long time (Plate 9, Figs. 73, 74, 77), usually being 

 still evident when the next division begins. This greatly facilitates 

 tracing the line of descent of each nucleus, and shows that nearly up 

 to the planula stage, at least, all divisions are clearly by mitosis. 



The deeply cleft or bilobed condition shown in Figure 70, which is 

 sometimes met with, is strongly suggestive of a double blastula. A 

 comparison with other somewhat similar stages leaves the impression 

 that this may be the result of an uncompleted first cleavage division; 

 it is as though the first division plane got but little beyond the condition 

 shown in Figure 58 (Plate 7) , and that then each half continued to seg- 

 ment independently. It is not impossible that in some cases this is 

 due to a mechanical restraint causing a bending in of one side. The 

 cleavage cavity is common to both halves, and entoderm cells are 

 being cut off from the deep ends of the blastula cells. It does not 

 seem probable, however, that such a blastula gives rise to a twin or 

 double embryo, for no embryos of that sort were ever found. 



d. Double Nuclei. — -In some of the cleavage stages the nuclei 

 are distinctly double and closely resemble conditions found by Hacker 

 ('95, :03) in copepods. In Tubularia this is very rare in the early 

 cleavages, one case only (Plate 7, Figs. 57a, 57b), at the end of the first 

 cleavage, showing more than a single vesicle. The cells of the blastula 

 stage, however, and of the embryo during the formation of the germ 

 layers as a rule have the nuclei double; indeed all the eggs of this 

 stage which were exainined showed this condition in some blastomeres. 

 While not all nuclei presented this appearance, the majority were 

 double and the apparent singleness in others was due, in some cases at 



