THE MOSAIC STAGE OF DIFFERENTIATION 265 



testes, presumably in the case of grafts from genetic males, since 

 testes may develop on either male or female hosts. When they 

 differentiate into ovaries, only a left rudiment will form a true 

 ovary ; the right rudiment will only develop the rudimentary ovarian 

 structure typical of the right ovary. This difference must be deter- 

 mined in relation to the original asymmetry gradient of the embryo. 



The different initial determination of right and left gonads is 

 further shown by another experiment. If all the germ-cells are 

 destroyed during the first half of the incubation period (which can 

 be accomplished by X-rays, owing to the high susceptibility of the 

 germ-cells to this agency) , the subsequent complicated differentiation 

 of the non-germinal portions of the gonad will continue, leading 

 to the formation, shortly before hatching, of testes, functional 

 (left) ovaries, and non-functional (right) ovaries, which are of 

 typical structure except for being sterile.^ 



Grafting and other experiments have also elicited other interesting 

 facts. In the first place, the grafting operation, and, still more, brief 

 exposures to low temperatures soon after visible sex-differentiation 

 has begun, favour the persistence of structures which normally 

 atrophy during development, such as the right Miillerian duct (ovi- 

 duct) of females, and both Miillerian ducts in males. The percent- 

 age of survival of such structures is raised by low temperature from 

 about 18 per cent, found in controls to over 70 per cent.^ 



Low temperature is known to inhibit or retard many develop- 

 mental processes: it would appear either that it has a specially 

 strong effect on processes leading to the reduction of organs, or that 

 since these processes, as shown by the 18 per cent, of persistence 

 in controls, are unusually labile, slight alterations in conditions 

 will cause large changes in their results (see also Huxley, 1932, 

 Chap. VI, 8). 



Another interesting fact is that the capacity of grafted portions of 

 the gonad-field for differentiation increases with their age when 

 grafted. Before the time of visible differentiation of germinal 

 epithelium, few or no grafts of this area give a gonad at all. A little 

 later, gonad-like bodies of uncertain^ sex are produced. If the graft 

 is taken still later, when it includes gonads which are well-formed but 

 still microscopically undifferentiated as regards sex, sex-different- 



1 Danchakoff, 1932. - Willier, 1932. 



