238 SCIENCE PROGRESS 



the right to say that the hereditary constitution of an organism 

 is entirely represented by these chromosomal units ? To this 

 we must answer that we do not as yet know, but that, again, 

 there is no fact which contradicts that assumption. 



It may be immediately objected that in the cytoplasm of 

 the fertilised egg we find at least two other sorts of differentia- 

 tion, both proved to be of importance for the process of develop- 

 ment, both obviously inherited in the simple sense of that 

 word. There is, first, a polarity, or gradient, stretching from the 

 more protoplasmic and active animal pole to the more yolky 

 and less active vegetative pole ; and, secondly, there is often an 

 accumulation of definite substances — " organ-forming stuffs " — 

 in definite regions of the egg, which have in many cases been 

 shown to play an important role in early development, certain 

 regions or structures of the embryo not being formed in the 

 absence of certain of the substances (Jenkinson, Experimental 

 Embryology, p. 223, etc.). 



It must, however, be remembered that the ovum is not a 

 simple undifferentiated cell, but highly specialised ; its polarity, 

 or axial gradient, appears probably to be due to the position 

 it has occupied during growth in the body of the parent (cf. 

 Child, Individuality in Organisms , 191 5), and the accumula- 

 tion of organ-forming substances in definite regions is a process 

 of differentiation comparable to any other process of differentia- 

 tion in any other cell. This being so, it is clear that the organ- 

 forming substances and their localisation may be properly 

 thought of as formed under the influence of more essential 

 parts of the cell — on our hypothesis, under the influence of the 

 gene-constitution in the chromosomes. 



If this is so, then such differentiations, like the subsequent 

 differentiations of the developing fertilised egg, take place as a 

 result of the constitution of the organism — the genes — in 

 reaction with the environment ; only they have been produced 

 precociously, in preparation for the rapid changes necessary in 

 early embryonic existence. 



It is, however, obvious that in one sense the cytoplasm is 

 part of the " constitution " — it is necessary to life. A nucleus 

 without cytoplasm cannot exist, and, even if it could, it pro- 

 bably could not form the specific cytoplasm out of itself, or 

 develop into a normal cell. What right have I, then, to speak 

 of the genes in the chromosomes, and not the cytoplasm, as 

 representing the essential constitution of the organism ? Both 

 appear to be specific, and both appear to be necessary to 

 existence. Much ink has already been spilt over this question ; 

 in order to spill as little as possible myself, I shall introduce at 

 once the conception which I believe we shall eventually come 

 to have of the chromosomal factors. Both cytoplasm and 



