566 A. C. WALTON 



' diminution' division of the first soma cell had been delayed, and 

 appears in its two daughter cells. In two of these emjbryos (figs. 

 75, 76) the stem cell (strp.) is shown with the fundaments of the 

 spindle forming around the centrosomes, but the nuclear mem- 

 brane is still intact. 



The six-cell stage (figs. 78, 79) shows the four soma cells, 

 descendents of the first soma cell, with resting nuclei, which 

 contain a small amount of chromatin. The chromatin lost at 

 'diminution' can be seen lying free in the cytoplasm. In figure 

 78 the metaphase plate of the stem cell is seen edgewise, and 

 figure 79 shows the appearance of the same plate when seen in 

 face view. No evidences of 'diminution' are ever found in the 

 stem cells. Figure 80 shows a six-cell embryo with normal 

 division occurring in the stem cell (strp.). amd a 'diminution' 

 division in the second soma cell (so.). By the division of these 

 two cells, the embryo passes to the eight-cell stage. Figure 81 

 shows a median section, which is perpendicular to the spindle of 

 the dividing stem cell, in which can be seen the difference be- 

 tween the resting nuclei of the soma cells and the nucleus of the 

 stem cell. The segmentation cavity has already made its ap- 

 pearance and continues to grow in size with the increase in the 

 number of blastula cells. A typical morula stage is not found. 



In embryos of two to four cells, four to eight cells, eight to 

 twelve cells, and sixteen to twenty cells, it was easy to distinguish 

 the soma cells produced by the first, second, third, and fourth 

 divisions of the stem-cell series (since these were undergoing 

 'diminution') from their sister stem cells, which showed no signs 

 of it. Each of the other soma cells, of course, possessed a nu- 

 cleus containing the diminished amount of chromatin. One 

 embryo containing forty to fifty cells showed the fifth soma 

 cell undergoing 'diminution' while its sister stem cell showed 

 normal division. In the few older embryos at hand, it was im- 

 possible to find evidences of the active process of 'chromatin 

 diminution;' but the two stem cells could be distinguished from 

 the soma cells by the condition of their nuclei, for the latter 

 were rich in chromatin while those of the soma cells were not. 

 No embryos were found which had advanced far enough to show 



