WEISMANN'S THEORY OF THE QERMPLASM 19 



spring ; that, in fact, the nucleus was the material 

 basis of heredity. 



Weisinann laid hold of this idea, but transmuted 

 it to fit in with his original theory of the germ- 

 plasm. Shortly put, his view is as follows : The 

 whole of the nuclear material is not hereditary 

 material, but only a definite part is such, and this 

 part, throughout the development of the individual, 

 remains unaltered in composition, and finally 

 becomes the starting-point for the generations to 

 come. The remaining and greater part of the 

 nuclear material does not remain in an unaltered 

 condition. The layers of cells, first formed in the 

 embryo, grow unlike each other, and give rise to 

 different organs and tissues ; Weismann draws the 

 inference that the nuclear substance as well alters 

 during the process of development, transforming 

 itself in a regular, orderly fashion, until, finally, 

 each different kind of cell in the whole body 

 contains a specific nuclearplasm. This segrega- 

 tion and transformation begins with the process of 

 cleavage itself, and thus ( the two daughter- cells 

 that arise from the first cleavage of the egg-cell 

 become different, so that the one contains all the 

 hereditary characters for the ectoderm, the other 

 for the endoderm. In further course the ectodermal 

 nuclear-plasm divides into that containing the 

 primary germs of the nervous system, and that 

 containing the similar constituents for the outer 

 skin. By further cellular and nuclear divisions the 

 inherited germs for the nervous system separate 

 into those for the sense organs, those for the central 



