304 bulletin: museum of comparative zoology. 



the cytoplasm in the region that marks the transition from it to the 

 deutoplasm. This condition is probably due to the breaking down of 

 the yolk spheres into coarse granules, which take a basic stain and are 

 definitely limited to this region of the egg. Opposite the equatorial 

 plane of the spindle these granules extend further into the cytoplasm 

 than in other regions (Figs. 81, 93). When the egg reaches the 

 metaphase, this condition disappears. An examination of the sections 

 adjacent to those figured indicates that the irregular area on the polar 

 (or formative) side of the young cleavage spindle is not a part of the 

 cytoplasm in which the active metabolic changes are taking place, but 

 is to be looked on as having a nuclear origin. An examination of the 

 female pronucleus in Figure 51 (Plate 8) shows that there is a great 

 deal more chromatin in the nucleus than is necessary to form the 

 chromosomes. The ring-shaped chromosomes, shown in Figures 87 

 (Plate 12) and 90 (Plate 13), were derived from solid masses of 

 chromatin, which during this ti-ansformation changed in its reaction to 

 stains. They cannot be individually followed to the chromosomes of 

 the cleavage figure. The division of the chromosomes is transverse 

 (Plate 12, Fig. 82), and they migrate to the poles of the spindle as in 

 maturation. In the late anaphase the chromosomes become vesicular 

 and fuse (Plate 13, Figs. 92, 91, 97), to form subsequently the quiescent 

 nucleus, as in the anaphase of the second maturation. 



The thickening of the interzonal filaments begins to appear in the 

 region of the equatorial plane during the anaphase ; this differentiation 

 arises not only on the interzonal filaments, but also on some of the rays 

 farther from the axis of the figure (Figs. 91, 92). As the cleavage 

 furrow deepens, it carries with it the central part of the interzonal 

 filaments and unites the thickenings on them into a single body, the 

 "Zwischenkorper" (Fig. 97). 



The centrosome is a solid body until after the metaphase (Plate 13, 

 Fig. 93 ; Plate 12, Fig. 82). In favorable sections the two regions, cen- 

 triole and centroplasm, can be distinguished in the anaphase, but the cen- 

 trosome in cleavage is so small that it is difi&cult to see precisely how its 

 differentiation takes place. Around the centrosome are the cortical and 

 medullaiy layers of the sphere, the medullary layer increasing in extent 

 during the late anaphase (Fig. 92). As the vesicular chromosomes fuse 

 to form the quiescent nucleus, the medullary and cortical layers become 

 diffused in the cytoplasm (Fig. 91). The centrosome in the late 

 anaphase is somewhat irregular in outline (Fig. 92). The centriole can 

 be seen in one of the centrosomes of this stage, but not in the other, and 



