EFFECTS OF CARBON DIOXIDE ON EGGS 365 



normalities in cleavage bear to the embryos which had been 

 allowed to undergo full development. 



Taking up the first question, a close examination of the material 

 has shown that the tetraster condition of the Si blastomere and 

 the irregular distribution of the chromatin in the A and B cells, 

 is due to the same cause, that is, the fusion of the chromatin 

 in the Si cell (figs. 2a to 2d). A glance at these figures will 

 show that in part of the eggs, only the ends of the chromosomes 

 were involved and that the middle portions were free. In such 

 eggs a division of the Siblastomeres occurs but, owing to the fused 

 condition of the chromatin, an equal distribution of it can not 

 take place. Diminution of the chromatin occurs and one 

 blastomere receives a number of small 'diminished' or somatic 

 chromosomes, while the other cell receives, in addition to the 

 somatic chromosomes, the whole mass of fused chromatin of the 

 equatorial plate. If the fusion involved part of the chromatin 

 which would normally go to form somatic chromosomes, then 

 one cell would receive this together with the waste chromatin. 

 When this fused mass goes to one cell, it does not undergo 

 degenerative changes but (probably because of the presence of 

 some somatic chromatin) it becomes resolved into a reticulum 

 and fuses with the normal nucleus of the cell. In this way, one 

 cell comes to contain more chromatin than its mate, and in later 

 stages, the cell with the least chromatin divides earlier. Vari- 

 ous stages of this process have been observed in my material. 



When, however, the fusion involved all of the chromatin, as 

 in figures 2b or 2c, then division appears not to take place. 

 Apparently, the fused condition of the chromatin is responsible 

 for this, but whether this prevented the centrosomes from going 

 apart, or whether the fused mass kept the cell wall from cutting 

 through, is not known. Stages that would decide this point 

 have not been seen, but various other steps in the process have 

 been observed. Thus the one cell comes to contain all the 

 chromatin which should be distributed between blastomeres 

 A and B. At the next division cycle, such eggs showed a tet- 

 raster in the Si cell and eight chromosomes are found in the 

 spindles (fig. D). 



