382 T. H. MORGAN AND UME TSUDA. 



the area where the blastopore is about to appear, so that a line 

 of dark pigment is distinctly seen in sharp contrast to the 

 lighter cells lying next to it. The change is so rapid that it 

 is often impossible to orient the sides of the egg. In some 

 cases, however, when the blastopore has only just appeared, 

 and before the pigment increases to any extent, it is easy to 

 see that the blastopore is forming on the previously lighter 

 side of the egg, as well as on the side which is segmenting 

 most rapidly. In spite of the dark pigment formed just above 

 the blastopore there is often a distinct light area on one side 

 of the arc. This area probably corresponds with the cell marked 

 b, the least pigmented cell, which lies opposite to the centre 

 of the pigment crescent and opposite to the dark cell, «, in the 

 diagrammatic figures A, B, C. 



Figs. X — XIII are views of favorable specimens, and show 

 distinctly a cluster of large cells, presumably those around the 

 lower pole. Fig. x is before the appearance of the blasto- 

 pore. The crescent-shaped area of pigment is distinctly seen, 

 the pigment coming much nearer the group of large cells on 

 one side than on the other. It is on this pigmented side that 

 the cells are largest. In the centre are a large cell and three 

 smaller ones, which probably are the four cells nearest the 

 lower pole. The unequal segmentation is also shown in 

 fig. XI, where the blastopore has already formed on the side 

 where the cell division is more advanced. In fig. xii there are 

 four cells distinctly larger than the surrounding ones, between 

 which probably run the first and second furrows. It will be 

 noted how much nearer the pigment approaches these cells on 

 the side marked m than on the opposite side, where the blasto- 

 pore appears. To the right and left of the blastopore the 

 pigment is less dense than on the opposite side, though it is 

 rapidly forming just above it. If it is granted that the four 

 cells are around the lower pole, and that this is the point 

 where the first and second furrows intersect, the exact rela- 

 tion of the blastopore to the lower pole can be easily ascer- 

 tained. Fig. xiii is a side view of the same egg, in which the 

 position of the supposed lower pole is shown. It is very near 



