ELEMENTARY MORPHOGENESIS 35 



half of the chromatin was thrown out of the egg by that 

 process : now this half is brought in again, but comes from 

 another individual. 



It is from this phenomenon of nuclear union as the 

 main character of fertilisation that almost all theories of 

 heredity assume their right to regard the nuclei of the 

 sexual cells as the true " seat " of inheritance. Later on 

 we shall have occasion to discuss this hypothesis from the 

 point of view of logic and fact. 



After the complete union of what are called the male 

 and the female " pronuclei," the egg begins its development ; 

 and this development, in its first steps, is simply pure cell- 

 division. We know already the chief points of this process, 

 and need only add to what has been described, that in the 

 whole first series of the cell -divisions of the egg, or, to use 

 the technical term, in the whole process of the " cleavage ' 

 or " segmentation " of it, there is never any growth of the 

 daughter-elements after each division, such as we know to 

 occur after all cell-divisions of later embryological stages. 

 So it happens, that during cleavage the embryonic cells 

 become smaller and smaller, until a certain limit is reached ; 

 the sum of the volumes of all the cleavage cells together 

 is equal to the volume of the egg. 



But our future studies will require a more thorough 

 knowledge of the cleavage of our Echinus ; the experimental 

 data we shall have to describe later on could hardly be 

 properly understood without such knowledge. The first 

 division plane, or, as we shall say, the first cleavage plane, 

 divides the eggs into equal parts ; the second lies at right 

 angles to the first and again divides equally : we now have 

 a ring of four cells. The third cleavage plane stands at 



