266 THE MOSAIC STAGE OF DIFFERENTIATION 



iation either in tlie male or female direction occurs in the grafts, and 

 its completeness and frequency (as well as the size of the resultant 

 gonad) increases with the age of the grafts. It would thus appear that, 

 as a result of processes occurring in the embryo as a whole, gonad- 

 detemiining substances tend to accumulate in liigher concentration 

 in the presumptive gonad-held during, and presumably for a short 

 time before, the stage of its early differentiation. This is perhaps 

 somewhat parallel \^-ith the intensification of organiser potencies in 

 the grev crescent area during the time from fertilisation to gastrula- 

 tion (p. 68) and with the progressive capacity* of the lens of the 

 bull-frog for self- differentiation after detemiination (p. 189). It 

 would be of great interest to discover whether a similar state of 

 affairs can be detected for other rudiments in other groups. A 

 parallel increase of differential potency in whole embr\os and 

 large fragments vni\ be discussed in the next section. 



The remarkable condition of the avian right ovary is correlated 

 with a marked regression of the cortex after its first formation. The 

 capacit}- for this must, as we have seen, be determined intrinsically 

 within the right gonad - ru diment ; but one of the results of this 

 primar}- differentiation as a right ovar\^ appears to be sensitivity^ to 

 substances emanating from the left oysltv ; for when the left ovary 

 is removed, the right hypertrophies. What it shall then produce is 

 determined bv the degree of degeneration which its cortex has 

 previouslv undergone. If considerable cortex is still left, this 

 dominates and it becomes a gonad of true ovarian type ; if less cortex 

 is left, both it and the medulla participate in the hypertrophy, 

 forming an ovo-testis. If the cortex had completely regressed, the 

 medulla hypertrophies and it forms a testis. The germ-cells 

 normally disappear from the right ovar}- in the first month after 

 hatching. If the hypertrophy takes place later, the resultant gonad is 

 sterile ; if earlier, as in ver\- early left ovariotomy, spermatogonia 

 may be formed. 



In conclusion, the special interest of the gonads for our purpose 

 lies in the fact that two alternative paths of differentiation lie open 

 to them, the actual path taken being normally first decided by the 

 sex-chromosome mechanism, and implemented by a quantitative 

 balance bet^veen two mutually antagonistic male-differentiating and 

 female-differentiating substances, locally produced by the two main 



