376 T. H. MORGAN AND UM6 TSUDA. 



nuclei were found in the yolk portion of the lower half of the 

 egg, which would normally have been separated from the upper 

 cells by the third furrow. 



In addition to the division of the cells of the segmenting 

 egg from the surface a delamination of the cells begins about 

 the thirty-second cell stage. The horizontal and vertical sec- 

 tions at this stage show elongation of the nuclei at right 

 angles to the plane of division. In the sixty-fourth cell stage 

 the delamination can be easily seen to have taken place by 

 the dissection of an egg under a hand-lens or a dissecting 

 microscope. 



A careful study of the segmentation of the cells around the 

 lower pole in the advanced segmentation stages has shown 

 that the greatest irregularity exists. 



In many cases, however — and I have reason to believe in 

 nearly all cases, — the cells lying nearest the lower pole, and 

 especially the four cells which are around the point where the 

 first and second furrows intersect, remain larger than the sur- 

 rounding cells. In the later stages of many eggs I have dis- 

 tinctly made out four cells, which I think are without doubt the 

 ones grouped around the lower pole. Figs, vii, viii, ix, where 

 cells marked b and a are much smaller than the surrounding 

 ones, might seem to oppose such a conclusion ; but in the later 

 stages, at the time of the formation of the blastopore (figs, x, xi, 

 xii), there is a certain regularity in the grouping of the larger 

 size cells around the point which, from other indications, 

 I should judge to be the lower pole ; and hence I believe that 

 such a cell as cell a, fig. viii, though smaller at this particular 

 stage than the cells surrounding it, does not develop so rapidly 

 later on. I have not been able to section the eggs at these 

 stages to find out what relation the real size of the cells bears 

 to the apparent size from surface views. At present I see 

 nothing against the hypothesis that the portion of the egg 

 around the lower pole in the late as well as in the early seg- 

 mentation stages is the most retarded portion of the developing 

 egg. The group of cells that remain largest always bear a 

 certain relation in position to the pigment that leads me to 



