THE CONSTITUTION OF THE HEREDITARY 

 MATERIAL. 



By T. H. morgan. 

 (Read April 23, 1915.) 



There are two ways in which the relation of the ^gg to the 

 characters of the individual that develops from the egg has been 

 interpreted. 



1. The egg has been thought of as a whole and the characters 

 of the individual as the product of its activity as a unit. 



2. The tgg has been thought of as made up of representative 

 particles of some sort that stand in a definite relation to the parts 

 of the individual that comes from the egg. 



Weismann, whose speculations occupied the forefront of interest 

 at the close of the last century, adopted the latter view ; namely, 

 that the germ is made up of particles, which he called determiners. 

 For Weismann embryonic development became merely the sorting 

 out of the particles of the germ to their respective parts of the 

 embryo. Each region of the body owed its peculiarities to the 

 particles that came to it by this sorting-out process. In fact, one 

 may go so far, I think, as to say that Weismann borrowed from 

 Roux this particular form of the preformation in order to give a 

 formal explanation of embryonic differentiation. But Weismann's 

 theory soon encountered three serious reverses. 



In the first place, the study of the minute structure and behavior 

 of the segmenting egg shows no evidence that any such sorting-out 

 process takes place, as Weismann postulated. It has been shown 

 that the chromosomes divide equally at every division, and that 

 every cell of the body contains the entire complex that was present 

 in the fertilized egg-cell itself. 



In the second place, it was shown that the sequence of the 

 cleavage planes of the egg could be artificially altered, yet a normal 

 embryo develop. 



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