ACCELERATION OF RATE OF CELL DIVISION. 351 



correlated with reduction of the pretrochal region. The pros- 

 pective character shown here will be found to apply also to other 

 characters of cleavage." 



Throughout the literature of embryology one finds an occasional 

 reference or allusion to the actual time that is consumed in the 

 various early cleavages, but many of these are rather incidental 

 and lack the character of systematic studies of the point. Of 

 those in which information of real value is given, Conklin's nota- 

 tions on the rate of cleavage in Cynthia may serve as an example. 

 The eggs of Cynthia were fertilized in a typical case at 5 o'clock 

 P.M. and the development carefully followed through. The actual 

 duration of the various stages was as follows : 



Fertilization at 5 P.M. to first cleavage (1-2 cell) 40 min. 



First to second cleavage (24 cell) 30 min. 



Second to third cleavage (4-8 cell) 20 min. 



Third to fourth cleavage (8 16 cell) 20 min. 



Fourth to fifth cleavage (16-32 cell) 20 min. 



Fifth to sixth cleavage (3664 cell) 20 min. 



Sixth to seventh cleavage (65-112 cell) '. . 20 min. 



Seventh to eighth cleavage (112218 cell) 20 min. 



Eighth to young tadpole 2 hours. 



Or, summarizing his observations, the period from fertilization 

 to first cleavage is 40 minutes, from the first cleavage to the be- 

 ginning of gastrulation 140 minutes, and again 140 minutes to the 

 fully formed tadpole stage. This is rather rapid development, 

 although in Molgula, another ascidian, the entire period is only 

 two thirds as long. 



The writer in 1914 recorded the average duration of the early 

 cleavages of Planorbis eggs under average temperature conditions 

 of the water of about 16-20 C. The durations of the periods 

 required for the completions of the first, second, third, fourth, and 

 fifth cleavages were, respectively, 1^2 hours, \y\ hours, ij4 hours, 

 i y\ hours, and i hour. 



For Hammed virescens, a typical case, recorded in connection 

 with the experiments reported herein, gave the following figures 

 for the durations of the first five successive cleavages : 85 minutes, 

 51 minutes, 59 minutes, 50 minutes, and 40 minutes. 



In all these cases the actual duration of time differs, but the 

 first cleavage is longer than those which follow it. 



