Eugene Howard Harper 365 



identified by its size and the appearance of the chromatin, as well as by 

 its surroundings. It has a well-developed double contoured membrane. 

 The contents are slightly plasmolyzed, a feature which has not been ob- 

 served to occur at any earlier stage. The chromatin is beginning to be 

 gathered into long threads. 



The First Division. — A later stage containing the prophase of the first 

 cleavage spindle is shown in Fig. 14, in horizontal section. The spindle 

 lies at the center of a small area free from granules (Fig. 28). Centro- 

 somes and asters are very indistinct, as would be expected at this stage 

 of division. The centrosome is not a deeply staining granule. The cyto- 

 plasm shows only indistinct radiations. The spindle is rounded at the 

 ends and rather broad. The chromosomes, sixteen in number, are partly 

 in the equatorial plate. None of them are yet splitting. The prevailing 

 shape of the chromosomes is that of a broad V. In the surrounding 

 protoplasm may be seen the same appearances as in the stage of the 

 segmentation nucleus (Fig. 14). There is an area of protoplasm whose 

 outer border is hyaline and the center surrounding the nucleus is more 

 granular. The area is elongated in the direction of nuclear division. Its 

 margin shows an appearance like that of outpushings. These are large 

 lobes, as if indicating an amoeboid movement of the whole mass. Curi- 

 ously enough, the granular interior conforms to the same outline, showing 

 lobes corresponding to the outer margin. Balfour, 85, states that " In 

 elasmobranchs before segmentation commences, the germinal disc ex- 

 hibits amoeboid movements." Here these amoeboid movements, if so the 

 appearances described are to be interpreted, are seen to be confined to a 

 region at the center of the germinal area whose diameter is about 0.5 mm., 

 or about one-sixth the diameter of the inner area of the disc. The area 

 of active protoplasm is differentiated into a more granular and a more hy- 

 aline pole. There are indications of a constriction which, if carried out, 

 would thus divide the cytoplasm of the area qualitatively. 



A stage of the first division is shown in Fig. 15, in which the nuclei 

 are separated a considerable distance. They are of quite small size. The 

 affected area of protoplasm shows a dumb-bell shaped figure and 

 the nuclei lie at about the centers of the two ends. The hyaline 

 outer border and the inner granular condition is still preserved. 

 The first furrow is being formed at the constriction, but is shallow and 

 does not appear in the section containing the nuclei. In the surface sec- 

 tion it is seen as a broad, shallow depression filled with cytoplasm of a 

 finely alveolar structure. Around the affected area lighter streaks may 

 be seen extending out into the surrounding protoplasm. One blastomere 

 is seen to be more hyaloplasmic than the other. 

 27 



