252 BERTRAM G. SMITH. 



position assumed insures an equal division of maternal and 

 paternal chromatin-groups in the ensuing mitosis. Thus the 

 second cleavage mitosis (Figs. 27 and 28) is in all essential 

 respects a repetition of the first, though the direction of the 

 spindle is different. Throughout the entire segmentation period 

 there is nothing in the behavior of the nuclei more impressive 

 than the precision and regularity with which the two nuclear 

 vesicles of a single nucleus are ranged side by side in the plane 

 of the equatorial plate in preparation for the formation of the 

 spindle. 



In the resting stage following the second mitosis the two nuclear 

 vesicles of each nucleus usually lie in the same horizontal plane 

 (Figs. 29, 30, 32) ; their entire orientation indicates their deriva- 

 tion from the two groups of chromosomes at one pole of the 

 spindle in the anaphase and early telophase of the preceding 

 mitosis (Fig. 28). Occasionally, though not so often as in the 

 preceding resting stage, one nuclear vesicle is found situated 

 above the other (Fig. 31). 



In Cryptobranchiis the third cleavage furrows are vertical 

 and intersect the second furrows at some distance from the first 

 (Plate IX., Fig. 45, J). Thus the third cleavage furrows are 

 nearly parallel to the first, but diverge slightly from it. 



In preparation for the third cleavage mitosis the two asters 

 take up positions on opposite sides of the nucleus, in such a 

 situation that a line connecting them is nearly parallel to the 

 plane of second cleavage (Figs. 33, 34; Plate IX., Fig. 45, H}. 

 In all cases the two nuclear vesicles entering the mitotic figure are 

 ranged equidistant from the two asters (Figs. 33, 34, 35), and 

 in a slightly earlier stage may be found rotating to this position 

 (Fig. 32). Thus in the third cleavage mitosis (Fig. 36) the 

 segregation and equal division of maternal and paternal chromo- 

 some groups is maintained. The direction of the axis of the 

 spindle is at right angles to the preceding mitosis, and parallel to 

 the first (Plate IX., Fig. 45, H and /). Upon the completion 

 of mitosis the third cleavage furrows form as described in the 

 preceding paragraph. At this time the four nuclei on the same 

 side of the second cleavage furrow sometimes lie precisely in 



