SEX DIFFERENTIATION 299 



the same part of the body. For example, there may be a syndrome between 

 limb and eye abnormalities. 



In considering this problem, however, we have not yet arrived at a com- 

 plete solution, for if differences between cells are the result of an external 

 stimulus we have yet to consider how the cells reproduce the differentiation 

 when they divide. How are these differences maintained? In the egg of Fucus 

 the stimulus is applied and then removed and a difference between two cells 

 arises. Then during subsequent cell divisions one cell gives rise to thallus 

 cells; the other to rhizoid cells. Thus the difference must be propagated or 

 reproduced. 



In the amphibian ectoderm, for example, the cytoplasm in the presump- 

 tive epidermis and in the presumptive neural plate is the same. The nuclei 

 are also the same. Then the stimulus of the organizer is applied. Some of 

 the ectoderm cells form neural-plate cells. Here one might say that the genes 

 interact with the nerve cytoplasm to give the differentiation of a nerve cell. 

 But when the primitive nerve cell divides, what insures that each of the two 

 daughter cells will become a nerve cell? The neural stimulus is no longer 

 present, and the environment now makes no difference. Therefore there is no 

 good reason why the nerve cell should reproduce itself. Some irreversible 

 change must have occurred in the cytoplasm or the nucleus, and this change 

 must be self-duplicating. Some substances must be changed by the initial 

 stimulus and thereafter reproduce themselves, very much like genes in the 

 nucleus. Thus the propagated change must be in the cytoplasm. 



The final picture, then, is that during differentiation a stimulus brings 

 about a formation or a release of a cytogene, a cytoplasmic gene, and this 

 self-duplicating unit, by reacting with the nuclear genes, maintains a new 

 type of cell. 



One of the most striking determinations of differentiation in the embryo 

 by genes is the determination of the sex of an individual. This is a differentia 

 tion which occurs in much the same manner as the differentiation of any 

 other structure in the body. The reproductive system of the primitive male 

 and female embryos develop in the same way for a period of time. For 

 several days of development in the chick embryo the same parts are present 

 in both male and female systems, and then somehow the genes induce or 

 stimulate differentiation toward the male and female direction respectively. 



In the case of the chick embryo sex differentiation is primarily in the 

 gonads themselves — in the ovary and testis. In the early chick embryo the 



