376 THEOPHILUS S. PAINTER 



neath; it is clearly undergoing diminution.) In figure 17 is a 

 later stage in which the fused P2 and EMSt cell is undergoing 

 division. It will be seen from the figure that there are four small 

 cells present which contain waste chromatin, besides two proto- 

 plasmic balls without any chromatin. The presence of a large 

 amount of chromatin in one small cell indicates that the four 

 small cells have arisen by the division of a tetraster. Compare 

 this egg with that shown in figure 15. The presence of the pro- 

 toplasmic balls indicates the same thing. One lies on the fused P2 

 and EMSt cells, and has arisen, probably during division, as in 

 figure 16. The other protoplasmic ball was undoubtedly formed 

 when the Si cell failed to divide. The interesting thing about 

 this egg is, that the cell which I interpret as coming from the 

 fusion of the P2 and EMSt blastomeres, such as we see in figure 

 16 is undergoing division; shows a tetraster and as may be seen 

 in the figure, the chromatin is undergoing diminution. Only 

 one case of this sort has been seen, but it has been very carefully 

 studied and there can be little doubt of the correctness of the 

 interpretation given. 



I do not propose to take up a detailed discussion of the ques- 

 tion, ''What causes the diminution in the somatic cells of 

 Ascaris?" And yet the question can not be fully omitted since 

 the observations recorded in the foregoing pages throw some light 

 on the subject, even though they do not give a final answer. It 

 is well known that two views have been held with regard to this 

 subject. Zur Strassen ('06) invoked what was essentially a 

 qualitative division of the chromosomes, in order to explain 

 the phenomenon. This view has been contested by Boveri 

 ('10), who, by his masterly analysis of dispermic and centri- 

 fuged eggs in Ascaris, showed that the explanation advanced by 

 Zur Strassen was untenable. Finding it impossible to explain 

 the cause of the diminution to factors residing in the chromosomes 

 themselves, Boveri turned to the cytoplasm and advanced his 

 'Schichtung' hypothesis. This author, convinced of the hetero- 

 tropic nature of the protoplasm in Ascaris, conceives of the 

 various substances being arranged in layers. The blastomeres 

 receiving certain layers of protoplasm, undergo diminution while 



