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, nope. 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 examined 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 
* 
‘ 
