A STATISTICAL STUDY^OF MITOSIS AND AMITOSIS. 23 1 



cells, and there is no morphological indication that the nucleoli 

 contain chromatin, as they never exhibit the radiations found so 

 frequently in the former group. As a consequence probably of the 

 absence of chromatin nucleoli and the other larger chromatic masses 

 seen in the nuclei of Fig. 6, such granules as one does find, are 

 slightly larger than the finely distributed chromatin of the nuclei 

 in the disintegrating cells. The two kinds of nuclei therefore 

 exhibit certain well-marked histological differences, and these 

 differences make it comparatively easy not to mistake the one 

 kind for the other — even in the transitional regions where both 

 occur together — regions which I eliminated altogether from the 

 determinations. 



Difficulties. 



The interpretation of the histological facts given in the preced- 

 ing section offers difficulties some of which inhere in the material 

 used, while others inhere in the subject, and would be met with 

 no matter what animal was studied. In the first place, the tech- 

 nical difficulties encountered in attempting to cut serial sections 

 were such that my series are only rarely complete and hence 

 unsuitable for the determination of the total number of nuclei per 

 embryo. I was able however to determine the total number of 

 nuclei in each section, and to count the resting ones and those 

 dividing either directly or indirectly. Each section was thus 

 treated as an independent entity without regard to what preceded 

 or followed it. The results therefore show that in the particular 

 set of sections which I studied, each one treated individually, a 

 certain number of nuclei were dividing directly and a certain num- 

 ber indirectly. The relative frequencies of mitosis and amitosis 

 are in no wise altered by the imperfections alluded to. 



The second difficulty that was encountered, was the physio- 

 logical differentiation of the entoderm into an anterior excretory 

 zone and a much larger posterior assimilative zone. While com- 

 plicating the problem to some extent, the regulative disintegra- 

 tion brought on by intense excretory activity, is restricted to a 

 very definite region, back of which nothing like it was ever ob- 

 served. It is necessary of course to conclude that some of the 

 entoderm cells are temporary larval structures, but this conclu- 

 sion should not be extended so as to include the entire entoderm. 



