74 A. RICHARDS. 



stage at which in general observations on the living eggs as to the 

 effect of exposure to the X-rays was closed. 



To sum up: the first division produces the cells AB, and CD; 

 the second division A, B, C, and D; the third division lA, la; 

 the next, which is the 12-cell stage, consists of 2A, la, 2a; the 

 i6-cell stage consists of 2A, la 1 , ia~, 2a; the 24-cell stage consists 

 of sA, la 1 , ia 2 , 2a l , 2a 2 , and 30,. 



EXPERIMENTS AND OBSERVATIONS. 



Eggs of Planorbis were exposed to the X-rays while immersed 

 in water in a watch glass. First they were carefully observed 

 under a microscope and the degree of their development noted. 

 The glass containing them was then placed at a distance of about 

 four inches below a tube of average hardness (after the current 

 had been started through the tube). An automatic regulating 

 device on the tube used made possible a fairly uniform exposure. 

 At the close of the exposure the eggs were again examined and 

 the degree of development noted. Observations were then made 

 at intervals during the day as convenience permitted. Usually 

 at the end of five or six hours the eggs had reached a stage where 

 further observation of them in the living condition was unprof- 

 itable. At this stage they were usually killed and fixed, although 

 some were allowed to develop for later study. 



In the cell divisions which are concerned in the maturation 

 and cleavage of Planorbis eggs up to the twenty-four cell stage 

 the writer has never observed any division under usual conditions 

 of temperature to take place in less than forty-five minutes. It 

 is, moreover, exceptional to find the division occurring in so short 

 a time as this, for in general the complete cycle does not take 

 place under an hour and often it is longer than that. Exact 

 data for a table showing just how long a period of time elapses 

 between each cleavage is not at hand, 1 but my records show no 



1 The following figures are taken from an observation made under average 

 conditions. They may be regarded as normal, but there is variation from this 

 norm as stated above. 



Experiment (i) The ist cleavage division required ij^ hours for completion. 

 2d cleavage division required i34 hours for completion. 

 3d cleavage division required i J4 hours for completion. 

 4th cleavage division required i J4 hours for completion. 

 5th cleavage division required about i hour for completion. 



