A STATISTICAL STUDY OF MITOSIS AND AMITOSIS. 24 1 



be doubled ; the second epidemic would yield 80 cells and the 

 third 160. A less severe epidemic, one having more probability 

 in fact, might exactly account for the approximately fourfold 

 increase observed. 



The assumption that epidemics of mitosis occur, but have been 

 overlooked, is unfortunately without foundation. In the larvae 

 used for the determinations of the relative frequencies of mitosis 

 and amitosis, as well as in the many others used as checks in 

 working out the more strictly embryological details, I have never 

 observed any indications of such epidemics. Similar indications 

 seem also to have escaped Osborn. 



It is impossible on the basis of such negative evidence as is 

 available to assert dogmatically that they do not occur. My 

 results however have some significance in this connection. A 

 comparison of stages III., IV., and V., shows that I found 2 

 mitoses, 42 amitoses, and 20 cells in stage III.; 1 mitosis, 28 ami- 

 toses, and 57 cells in stage IV.; and o mitosis, 41 amitoses, and 

 93 cells in stage V. It might be asserted that the divisions 

 which account for the increase from 20 to 57, took place during 

 an epidemic of mitosis at some time between stages III. and IV. 

 It might be claimed also, that the increase from 57 to 93, had 

 come about as the result of a similar epidemic between stages 

 IV. and V. These stages were selected because their external 

 characteristics mark definite steps in the acquisition of the adult 

 body form. Stage IV. is approximately a half-way station be- 

 tween stages III. and V. It is important therefore that in other 

 respects also the larvae should be half-way between the two ex- 

 tremes, for this is at least an indication of an even tenor in the 

 rate of all of the developmental processes. Were growth spas- 

 modic and not uniform, it would be very curious that the number 

 of entoderm cells in a corresponding cross-section of the " half- 

 way" larva should be 57, for the mean between 93 and 20 is 



56.5. 



The view that growth is uniform in rate gains in validity when 

 we consider the percentage of indirect and direct divisions which 

 occur during this crucial period. For stage III. the former is 

 .2 per cent, the latter 4 per cent.; for stage IV., the former is .1 

 per cent., the latter, 4 per cent.; whereas, for stage V., we have o 



