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 (c/. po/.) 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 still occur. In such cases the indications of a cleavage 

 cavity are not so frecpient; still, as Figvire 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. 



c. 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 



