94 THE BIOLOGICAL PROBLEM OF TO-DA Y 



the direction of the cleavage-planes, the relative 

 positions and the different sizes of the cells exhibit, 

 under normal conditions, the most marked regu- 

 larity. But it may be shown directly that this 

 regularity is not the result of special determinants 

 lying within the nucleus. For ail these phenomena, 

 which are characteristic in the cleavage of the 

 frog's egg, as well as in the cleavage of ail other 

 eggs, are determined directly by the qualities of 

 the yolk surrounding the nucleus. 



In several publications I have shown clearly that 

 the external form, of an egg and the arrangement 

 of its contents, according to the different specific 

 gravities of the component particles, determine the 

 position of the nucleus and of the successive planes 

 of division. Similarly, the different sizes of the 

 ceils first formed and the unequal rate of division 

 shown at the two poles of the egg depend upon 

 the constitution of the yolk, upon the cleavage of 

 the yolk into a portion richer in protoplasm and a 

 portion poorer in protoplasm, and upon the differ- 

 ences in the bulk of protoplasm that in this way 

 reaches each of the first-formed cells. 



In many cases it has been shown that there is a 

 constant relation between the first three cleavage - 

 planes of the egg and the long axis of the animal 

 that arises from the egg. Weismann and Roux 

 make this a proof that, in nuclear division, the 

 nuclei that arise have different qualities ; that the 

 protoplasmic masses lying to the right and left of 

 the median plane are set apart to build up the 

 right and left halves of the embryo ; that, similarly, 



