SEX DIFFERENTIATION 297 



body but that a specific gene expresses itself or acts only in the presence of a 

 chemical environment which is specific for that gene. Thus a gene may bring 

 about red eye color but the body may be yellow. The eye would then have 

 the specific chemical compounds for red-pigment formation, and the rest of 

 the body would lack one or more of these compounds. 



Logically this brings us back to the question of what factors make the 

 eye different from the remainder of the body. What produces this specific 

 chemical environment for gene action? Here the argument becomes some- 

 what attenuated, because one can say that genes which have acted previously 

 make the eye different from the tissues of the rest of the body. As a result 

 the red gene will act in the eye region exclusively. However, these previously 

 acting genes must also have had a different chemical environment in which 

 to express themselves, or else there would be eyes all over the body. Thus we 

 are led into a very involved argument. 



It is easy to see that this line of approach carries the problem back finally 

 to the very first differences in the egg. For if the genes are all alike in the 

 early egg nuclei, the only way one can obtain differences in development is 

 through the differences in the cytoplasm. That is, differences in the cytoplasm 

 react with the same genes to produce different cell types. 



How do the first differences arise in the cytoplasm of the egg? In most 

 cases the eggs which we study show these differences as early as the eggs are 

 obtained. There is one egg, however — that of the alga Fucus — -in which 

 there apparently are no initial differences in the cytoplasm of the egg. The 

 polarity of this egg is subject to many environmental factors, such as light, 

 heat, hydrogen-ion concentration, to name but three of them. Any one of 

 these factors affecting one side of this egg determines the whole future 

 development of the egg (Fig. 191). The first cleavage of the egg is very 

 unequal. There is an outgrowth on one side of the egg, and the first cell 

 division cuts off this outgrowth into a cell. This cell becomes the rhizoid, and 

 the other cell forms the thallus of the alga. Here we have a situation whereby 

 an external stimulus creates a difference, which we can symbolize by T and R, 

 within the cytoplasm of the egg. At the first cell division there is this differ- 

 ence in chemical composition, symbolized by T and R, reacting with nuclei 

 which are identical. And the reaction leads then to a difference in the develop- 

 ment of these two cells. This throws the initial difference in an egg back to 

 some environmental factor, perhaps in the ovary while the egg is developing. 

 The two sides of the egg may be subjected to different chemical environments, 



