206 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
chromatin. ‘The entire dissolution of the nucleolus within the germi- 
native vesicle before the chromosomes form, suggests the possibility 
that the nucleolus may contain something necessary for the formation 
of the chromosomes. 
2. Polar cells.— In anticipation of polar-cell formation the germi- 
native vesicle at the periphery of the egg becomes reduced in size 
_and ovoidal, and the chromatin begins to concentrate into larger 
masses. ‘I'wo polar cells are formed, both by mitosis. 
3. Cleavage.—'The segmentation is total, unequal and often 
irregular. Polar cells, first cleavage furrow and nuclei of the first 
formed blastomeres occur at the same pole of the egg. The division 
of nuclei may be followed at once by segmentation of the egg, or 
nuclear proliferation may take place for some time before any cyto- 
plasmic division occurs. ‘These conditions are only extremes of a 
series and not sharply separated from each other. The cleavage may 
be considerably modified because it takes place within a closed gono- 
phore. 
Segmentation results in a blastula with a definite segmentation 
cavity, which may, however, be reduced to a few small spaces between 
the blastomeres, or even disappear altogether. ‘There is a multipolar 
delamination of the blastula cells, and the segmentation cavity becomes 
filled up by a mass of cells which represent primary entoderm, the 
superficial cells representing primary ectoderm. The so-called morula 
stage in reality corresponds to the end of the formation of the germ 
layers, which assume their definitive positions and relations by a later 
specialization and rearrangement of cells. 
4. Double Nuclei.— It has been possible to trace a direct descent 
of nuclei from the first cleavage to the germ layers, and in all cases the 
division is by mitosis. ‘The nuclei of the blastula and of the germ 
layers are usually double, consisting of two distinct vesicles, each with 
a nuclear reticulum and one or more nucleoli. Chromosomes form 
independently but synchronously in each half and may be in two, more 
or less distinct, groups in the equatorial plate of the spindles. Each 
daughter nucleus re-forms at once into two vesicles. This condition 
may represent an autonomy of the sperm and egg chromosomes, but 
the absence of double nuclei in the cleavage stages previous to the 
formation of the blastula seems to be fatal to this view. Possibly this 
condition may be connected in some way with an intense nuclear 
activity. 
