WEISMANN'S THEORY OF TTtE GERMPLASM 93 



They are secondary formations, that can arise only 

 after the multiplication of cells, and from the 

 varied combination of cell-characters that accom- 

 panies the multiplication of cells. 



In the foregoing pages I have attempted to prove 

 the untenability of the doctrine of determinants 

 from general considerations. I shall now attempt 

 the same by analysis of a concrete case. The frog's 

 egg may serve for this. It is a familiar object, 

 frequently studied. Consider its mode of division, 

 and the formation of the blastula, gastrula, and 

 germinal layers. 



In cleavage the nucleus plays the chief part, and 

 thus has been accepted as the bearer of the 

 hereditary mass. But no single, special determinant 

 gives the impulse for cleavage ; rather, the co- 

 operation of all the particles that are essential to 

 the nature of the nucleus. The chromosomes, 

 which we may regard as independently growing 

 and dividing units, must have doubled by assimila- 

 tion of food material from the yolk ; perhaps, also, 

 the centrosorne may have doubled in the same 

 way before the nucleus is in a condition to divide. 

 This condition itself appears the necessary result 

 of many different processes of nutrition and growth, 

 as the result of complicated chemical processes that 

 run their course within the separate, elementary, 

 vital units of the nucleus. 



The multiplication of the nucleus into two, four, 

 and eight daughter-nuclei, and so forth, gives the 

 impulse for the breaking up of the yolk into a 

 corresponding number of cells. In that process 



