HARGITT: PENNARIA TIARELLA AND TUBULARIA CROCEA. 191 
result in a blastula of precisely this character is probably due, in part 
at least, to pressure consequent upon development within a closed 
gonophore. ‘The further division of cells accompanies the process of 
the formation of the germ layers. 
The proliferation of nuclei, not followed at once by cytoplasmic 
division, was somewhat more common in eggs collected in the spring, 
though this probably has no especial significance. In this segmenta- 
tion there were no signs of nuclear reorganization from a previously 
fragmented nucleus, and no signs of amitosis, the evidences of nuclear 
division always pointing to mitosis. Figures 64 and 65 are outlines 
of two different eggs with all nuclei projected on the same section by 
the superposition of carefully made camera drawings. Four pairs 
of nuclei are shown in Figure 64, and their origin from a single nucleus 
is not hard to conceive. ‘The polarity of the egg previously noted is 
well shown here, the nuclei being located on the convex side of the egg, 
the one opposite the spadix. A polar cell (cl. pol.) is shown and the 
cleavage of the cytoplasm has begun, one furrow having separated off 
a small blastomere (shown by dotted lines), and another furrow having 
started from the region of the polar cell. In Figure 65 fourteen nuclei 
are shown; one blastomere is entirely separated and a second furrow 
has made its appearance. ‘The absence of polar cells makes an exact 
orientation of the egg impossible, though the aggregation of the nuclei 
on the side opposite the spadix is nearly as pronounced as in the pre- 
vious figure. 
A rather late stage of cytoplasmic division is shown in Figure 67, 
though there are still many more nuclei than blastomeres, and the 
inequality of the blastomeres is very evident. In Figure 68 is shown 
a still later stage, in which the cells are more nearly equal, though 
variations in size stilloccur. In such cases the indications of a cleavage 
cavity are not so frequent; still, as Figure 67 indicates, a small cavity 
does occur. In Figure 68 there is no cleavage cavity, merely a solid 
mass of cells, but it is not impossible that at an earlier stage this, too, 
had a cleavage cavity, which became obliterated during subsequent 
cleavages. ‘The embryos resulting from this kind of segmentation 
apparently differ in no way from those formed by the first method 
described. Figure 66 shows a section of an egg from the same hy- 
dranth as that of Figures 64 and 68; here the nuclear divisions have 
been followed at once by cleavage of the cytoplasm. 
ce. ‘The Formation of the Germ Layers.— The stage shown in . 
Figure 63 is near the beginning of the process of delamination which 
ends in the separation of entoderm from ectoderm. At this stage 
