204 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



vegetative. At about 9.30 p. m., when I began these drawings, the nuclei were in kary- 

 okinesis ; at 10 p. m. nearly all were in the diaster stage (as represented here) ; at 10.10 

 p. m. superficial furrows began to appear, separating the daughter cells in the region of 

 the animal pole. At 10.15 p. m. cell cleavage was completed. The cells on the opposite 

 side of the egg lagged somewhat behind the others, the cleavage furrows in that 

 region being completed about 5 minutes later. The yolk segments, both before and 

 after division, were wedged rather closely together and were now polygonal in outline 

 over the greater part of the egg. At 11 p. in. this egg had the appearance represented 

 in fig. 221, when seen from the animal pole. The yolk segments or hillocks now 

 protruded, becoming very convex, and the whole egg took on a beautiful mulberry-like 

 appearance, the segments which were visible to the naked eye being dark green with 

 whitish protoplasmic caps or centers. At 1 a. m., or 2f hours after the last cleavage 

 period was completed, the segments flattened down, and by mutual pressure assumed 

 a polygonal form, the energy which had been stored up during the interval being now 

 directed to do the work of the next cleavage. 



A similar phase is illustrated by cuts 23, 24, plate G, the former showing the 

 vegetative pole. When these drawings were made, at 12.55 p. m., the nuclei were 

 mostly in the diaster stage of division, and in 70 minutes cleavage furrows were 

 beginning to appear. 



An egg in a stage quite similar to that seen in fig. 221 is represented in fig. 222. 

 When first observed, at 10 a. m., from thirty-six to forty segments were visible over 

 that half of the egg corresponding to the animal pole. At 10.55 the nuclei were in 

 active division. At 11.30, when the drawing was completed, cell-cleavage furrows 

 were beginning to appear, and in 20 minutes the segmentation was completed over 

 the greater part of the surface. At 12 m. (30 minutes from the time cell-cleavage 

 became visible, and 65 minutes from the beginning of karyokinesis) the process was 

 complete and the segments had begun to swell. The egg in this phase is represented 

 by fig. 223. At 2.45 p. m. active karyokinesis again began, and at 6.25, or in less 

 than 4 hours, division of the segments was again completed. This phase of the 

 segmentation lasted nearly four times as long as the former period. The drawing of 

 it (fig. 224) was made at 9 p. m., and represents the side including the vegetative 

 pole. (See figs. 218, 219, and 220.) The polygonal cells, near the central part of the 

 area represented, were the last products of this segmentation. 



The time occupied in cell division is illustrated by another egg, which was under 

 observation 7 J hours (10.55 a. m. to 6.25 p. m.). It was of about the same age as the 

 egg shown in fig. 222. At 10.55 a. m. the nuclei were in the diaster or metakinetic 

 stage of division. At 11.40 a. in., or 45 minutes later, cell division or segmentation was 

 completed. At 1 p. m., 80 minutes later, the superficial furrows were very definite and 

 the protoplasmic cap of each segment was more distinct when examined in reflected 

 light. At 2.45 i). m., 105 minutes later, or nearly 4 hours from the beginning of the 

 last period of segmentation, the segments were closely crowded and nuclei were again 

 in active division. (Stage of the equatorial plate.) At 4.15 p. m., 1J hours later, 

 cleavage amphiasters were formed, but no furrows. At 6.25 p. m., 2 hours and 10 

 minutes later, or 3 hours and 40 minutes from the time of appearance of the equatorial 

 plate, segmentation was completed. Here the total segmentation period lasted about 

 6 hours and 45 minutes, of which 2 hours and 20 minutes were spent in quiescence 

 and 4 hours and 25 minutes in activity. 



