246 SCIENCE PROGRESS 



the segmenting ovum divides. But this same gene-complex 

 will be lodged in a different environment according as it is in 

 a cell of the animal or vegetative half, according as that cell 

 contains a particular organ-forming substance or not. The 

 same material in different environments will give different end- 

 products. These can then interact with each other, or with 

 other elements of the constitution ; one part may be exposed 

 directly to the surrounding medium, as in the ectoderm, another 

 part, like the mesoderm, may be exposed only to an internally- 

 secreted medium. 



Given, in fact, any diversity of different regions of the egg- 

 cytoplasm, whether pre-existing or induced after fertilisation, 

 and a formal explanation of differentiation becomes possible. 



This is of prime importance to us, for it answers an objection 

 raised by many critics. If the constitution of an organism, 

 they say, is to be sought in the chromosome- gene complex, 

 and if this is divided into identical halves at each mitosis, then 

 every cell in the body will have the same chromosome constitu- 

 tion. How, therefore, can differentiation be possible ? Such 

 objectors, often physiologists, are falling into the very error 

 which they pretend to deplore : they are thinking in unreal 

 symbols. When the geneticist affirms that the constitution 

 of an organism is composed of a number of unit-factors, he 

 merely epitomises the ascertained fact of his own experiments. 

 But, in that assertion, he states nothing as to the physiology 

 of their development. When he states that he knows a pair 

 of Mendelian genes, let us say X* and X, which determine yellow 

 and agouti colour respectively in mice, he in reality asserts 

 only that, without X or X^, the mouse will not be either 

 yellow or agouti ; with X and without X', it will be agouti and 

 not yellow ; with X*, it will be yellow. 



If, further, the result of his experiments indicate that these 

 factors are lodged in the chromosomes, this contributes simply 

 another fact for the physiologist to explain. If the explana- 

 tion be not immediately forthcoming, this is no reason for deny- 

 ing the facts, but only for further thought and research. 



As a matter of fact, it should not surprise us in the least 

 to find the identical gene-mechanism in every cell of the body. 

 The mechanism of a dozen Broadwood pianos may be identical, 

 but the melodies elicited from them may and will differ with the 

 executant. The executant is for us represented by the environ- 

 ment, whether intra-cellular, intra-organismal, or external ; 

 the construction of the pianoforte by the gene-complex, and the 

 different melodies by the various types of differentiated cells 

 of the adult body ; of the genes in any one cell, some will 

 remain dumb, some never sounded ; in one cell a gene will be 

 working at full activity, in another partially inhibited, in a 



