34 TADACHIKA MINOURA 



was very variable. One graft would grow readily; while another 

 would become partly necrotic. Another factor to be considered 

 is the condition of the embryo on which the graft is implanted. 

 Even if the grafts grow equally well and remain upon the embryos 

 an equal length of time, they will not necessarily produce the 

 same degree of effect, since the embryos themselves vary as to 

 age, and presumably also in general metabolic condition, degree 

 of resistance to foreign tissues, and so on. In consequence of 

 these variable factors, it is difficult to determine in experiments 

 of this kind the relation between amount of dosage and degree 

 of modification of the embryos. In general, however, it may 

 be said that grafts must remain upon the embryo for a certain 

 minimum length of time in order to bring about a modification 

 of its sexual characters. This minimum length of time is about 

 one week. 



d. Effect of age of birds furnishing the grafted gonads. Another 

 point of interest which arises in connection with these experi- 

 ments is whether the age of the individuals from which the gonads 

 to be grafted were taken bears any relation to the degree of 

 modification produced. The age of the birds whose gonads were 

 removed for grafting varied in these experiments from embryos 

 eight or nine days in age up to adult fowls. It has seemed to me, 

 however, that in many cases, gonads obtained from young chicks, 

 between the ages of one week and one month, were more effective 

 in producing modifications in the embryos on which they were 

 grafted than gonads from birds of other ages. 



e. Effect of the age of the embryo at the time of grafting. The 

 experiments yielded definite results upon this point. As has 

 already been stated in the section dealing with the growth of the 

 grafts, the best results were obtained when the grafts were 

 implanted on embryos during the second week of incubation; 

 that is, from the seventh to the thirteenth day. The reason for 

 this fact is not difficult to find. In the chick embryo the differ- 

 entiation of the sexes appears on about the sixth or seventh day 

 of incubation and progresses rapidly to the eleventh and twelfth 

 days. This period, approximately the second week of incubation, 

 is then the period of active sexual differentiation. It is char- 



