82 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



cavity may attain a volume equivalent to one half or two thirds that of 

 the undivided ovum. Throughout its whole history, from its inception 

 to its culmination, both in the living egg and in preserved material, 

 the cavity is always sharply marked off from the protoplasm of both 

 cells. This is true, no matter what may be the point of view from 

 which the egg is observed. Of course the boundaries of the cavity are 

 indistinct where they lie oblique to the optical axis of the microscope, 

 but there is always one focal plane at which the limiting " membrane " 

 of the bounding cells sharply and distinctly separates the protoplasm 

 from the fluid contents of the cavity, and moreover there is no trace 

 whatever of any shading off of the protoplasm toward the cavity such as 

 Stauft'acher ('93) describes in Cyclas. We are therefore compelled to 

 conclude that the cavity is distinctly intercellular. 



The maximum development of the cavity is followed by a forcible 

 expulsion of its contents. This takes place suddenly, and the elimina- 

 tion of the fluid may be total, or only partial. After a total elimination 

 the egg resumes its original size, and tends to take again the spherical 

 form. In case of a partial expulsion the cavity retains its polar diameter, 

 but the antero-posterior diameter is reduced (Plate I. Figs. 5, G). In the 

 egg represented in Figure E, the spindles leading to the third generation 

 were present when the cavity of the two-cell stage had reached its max- 

 imum. It often happens that two or even more expulsions of the liquid 

 contents of the cavity occur between the two- and the four-cell stages. 

 In the series represented in Plate I. Figs. 1-7, drawn from the same egg 

 kept under continuous observation, the two blastomeres had attained the 

 maximum divergent or rounded condition at 11.30 a. m. At 12.35 P. M. 

 they had reached the flattened or coalescent condition (Figs. 1, 2), and 

 at 12.45 p. m. the central cavity had appeared (Fig. 2). This increased 

 in size (Figs. 3-5), reaching a maximum at 1.30 p. m., when a partial 

 expulsion occurred (Fig. 6). After this expulsion the nuclei could no 

 longer be seen distinctly in the living egg. The cavity again increased 

 in volume (Figs. 6, 7), and at 2.02 p. m. a second and total elimination 

 took place coincidently with the division into four cells ; this was accom- 

 plished, i. e. the furrows had reached the vegetative pole, at 2.05 p. m. 



It is not always possible to determine the point at which the fluid 

 contents are ejected. When a sudden reduction in the size of the cavity 

 is noticed, there is sometimes visible in the albumen adjacent to the 

 cleavage furrow a small sphere of transparent matter differing in its 

 refractive index from the surrounding albumen into which it very quickly 

 merges. When, however, the reduction in the size of the cavity is 



