II CLEAVAGE 61 



differentiation in the cytoplasm, for the simple reason that 

 that differentiation exists before cleavage takes place. 



We may begin this discussion by a re-examination of the 

 facts on which Roux's own theory was based. 



The first statement is that the first furrow in the Frog's egg 

 always coincides with the sagittal plane. This is certainly 

 not the case, for if a sufficiently large number of eggs be 

 examined it will be found that while in the majority the 

 angle between the two planes is small, it is nevertheless 

 possible for that angle to have any value. 



One example will be enough. The following table gives 

 the frequencies for different values of this angle under the 

 most favourable circumstances, that is when the disturbing 



* O 



influences of mutual pressure and gravity have been removed. 



TABLE XI. 



Angle between 

 First Furrow and Sagittal Plane. Frequency. 



-90 -75 6 



75 -60 10 



60 -45 20 



45 -30 31 



30 -15 70 



15- 82 



-f 0-15 102 



15 -30 50 



30 -45 31 



45 -60 9 



60 -7 5 7 



75 -90 10 



Total 428 



The tendency of the two planes to coincide is measured by 

 the standard deviation (cr), which in this case is 3145 + 0-73. 



If now under the same conditions the standard deviation of 

 the angle between the plane of symmetry (plane of the grey 

 crescent) and the sagittal plane be determined it is found to 

 have a value of 26-80 + 0-82. In other words there is a greater 

 tendency of the sagittal plane to coincide with the plane of 

 symmetry than with that of the first furrow. The standard 

 deviation of the angle between the plane of symmetry and 

 the first furrow is highest of all, namely 3446 + 1-07. 



The relation between these planes may also be stated by 



