WEISMANN'S THEORY OF THE GERMPLASM 99 



The conditions of gastrulation and of the forma- 

 tion of the germinal layers are similar. The 

 invagination of the blastosphere comes about by 

 the co-operation of all the cells of its wall, by 

 local differences in the rates of growth in that wall, 

 from dissimilarities in its curvature, from many 

 causes which have not yet been sufficiently sought 

 out and investigated. As cell division itself depends 

 not upon special particles, but upon changes in the 

 entire nuclear contents, it follows that the growth 

 of the blastosphere- wall, which is merely the sum 

 of the growth of all the cells in it, cannot be 

 determined by special groups of determinants. 



As an attempt to explain gastrulation, the origin 

 of the germinal layers and many other events of 

 development, the doctrine of determinants has re- 

 versed cause and effect. Certain cells do not 

 become invaginated into the segmentation cavity 

 because they possess groups of determinants that 

 impel them to the assumption of inner layer 

 characters. The reverse is the truth. Local condi- 

 tions of growth cause the invagination of a set of 

 the cells of the blastosphere-wall. This invaginated 

 layer of cells, brought into a new position with regard 

 to its environment, becomes the endoderm and re- 

 ceives the stimulus to assume the character appro- 

 priate to the new environment. It is unlogical to 

 speak of endoderm in the fashion of many text- 

 books and treatises on embryology, while the so- 

 called endoderm cells still form part of the outer 

 surface of the blastosphere, or even while they are 

 still in process of formation by cleavage. For 



