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. Two 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. 



