OKLAHOMA ACADEMY OF SCIENCE 27 



But it is this fact in connection with the ribbon like egg case that 

 renders them desirable for experimental purposes. In conducting 

 the experiments a ribbon would be cut into segments one or more 

 of which would form a control, while the others would be placed 

 in the various solutions as desired and the results noted in com- 

 parison to the control. 



Egg laying takes place usually about the time of the first 

 light, and it is probably that the light acts as a stimulus to the laying 

 process. There are exception to this rule, but they do not seem to 

 be of special significance. 



The duration of the early cleavages is illustrated by one typical 

 case which was followed through in detail. The observations were 

 made at 21.5°C. 



One cell stag'; first observed at 6:45 a. m. 



Maturation completed at 7 :45. 



First cleavage (2 cells) completed at 9:10, an interval of 1 hr., 25 min. 



Second cleavage (4 cells) completed at 10:01, an interval of 51 min. 



Third cleavage (8 cells) completed at 11 :00, an interval of 59 min. 



Fourth cleavage (12 cells) completed at 11 :50, an interval of 50 min. 



Fifth cleavage (16 cells) completed at 12:30, an interval of 40 min. 



From this and other data it appears that the early range in dura- 

 tion is from eighty minutes down to forty minutes. The veliger stage 

 is reached at the end of about 48 hours. Hatching begins on the 

 fifth day usually, and is completed in two or three days more. 

 The percentage of embryos which hatch is very large unless some 

 external factor interf ers ; it is seldom less than 85% and I have 

 often seen from 98% to 100% reach this stage. 



XIV. THE ACCELERATION OF THE CLEAVAGE RATE 



OF HAMINEA VIRESCENS 



A. Richards. 



From the Zoological Laboratory of the University of Oklahoma. 

 Contribution No. 16, Second Series. 



Cell division is controlled by factors which are not clearly 

 understood, nor is it to be expected that they will easily be ex- 

 plained because of the great complexity of the processss involved in 

 mitosis. One method of analysing the phenomena is that of modify- 

 ing the process by various means. The present paper is an account 

 of attempts to modify the rate of cleavage in the eggs of the 

 gasteropod, ?Ja minca. 



There are many factors known which act to retard cell division 

 in tissues of many kinds. Indeed one of the immediate effects 



