116 ANNALS NEW YORK ACADEMY OF SCIENCES 



of the body, as when one eye of a mouse is black and its mate is pink, or 

 one wing of Drosophila is long and the other short. 



It may therefore be worth while, perhaps, to attempt to recast our 

 conception of development in the light of these facts. Obviously it may 

 not be necessary to attempt to explain all embryonic phenomena as due 

 to segregation. There are several facts of experimental embryology that 

 seem to indicate that there are other relations between the cells that are 

 important factors in determining their fate, but the essential facts of 

 specification and differentiation may nevertheless be explained by the par- 

 ticulate theory. No fact is more evident than that in most cases a region 

 once determined has lost its power to produce other parts, and this fact 

 bears a strong resemblance at least to the segregation process in the germ 

 cells that produce recessive characters. 



This argument may seem to lead back to Weismann's theory of develop- 

 ment, but it differs from his view in two essential respects: Firstly, it 

 admits that development involves not only a process of separation of 

 particles, but that other factors also are involved, especially those that 

 regulate the symmetry of the body and its parts. Secondly, it does not 

 look upon the particles of chromatin as each representing a different part 

 of the body — of its characters, in short — but for the development of 

 each part certain parts of the complex of particles form the necessary 

 foundation and for other parts other complexes. The germ material from 

 this standpoint loses in the course of development now one, now another, 

 of its materials, and the absence of these materials is responsible for what 

 each region can produce. Weismann's view postulates the presence of 

 particular particles that produce each part. On his view, each part has a 

 representative in the original germ plasma, while on the view here sug- 

 gested, most of the entire material is necessary for the development of 

 ^ach part, but the loss of one or another particle of the chromatic com- 

 plex turns the developmental processes into different channels. I am 

 aware that speculation concerning the processes of development and 

 Mendelian segregation may seem premature at present, but I can not but 

 think that it may be worth while to attempt to bring under the same 

 point of view the process of gametic segregation and the processes that 

 take place in the development of the embryo. 



