322 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



in the same direction (Fig. 14, b). The resuh of this rather complex change is a 

 crescent shaped zone which is either of a Hghter tint or distinctly grey, because 

 there is an inner yolk wall covered by a thin veil of melanic pigment. The region 

 endowed with this grey or lighter crescent will be the site of the blastopore, and 

 is called dorsal. In Urodeles, such a "clear" crescent is simply due to a general 

 reaction of the cytoplasm, without any apparent relation to the place of fertili- 



Fig. 13. Displacement of tiny electrolytic marks placed on the 

 pellicle of a frog egg. The dotted area is the region where the 

 grey crescent will later appear. For each of the marks, the cross 

 indicates the initial location, the arrow, the point attained. Note 

 the direct upward movement of the marks placed in the medio- 

 sagittal plane, the convergence of the others toward this plane, 

 a. p., animal pole; v.p., vegetal pole. From Ancel and Vintem- 

 berger, 1948. 



zation. In frog eggs which have been activated by pricking, the same absence of 

 correlation holds true. However, in normally fertilized frog eggs, the grey crescent 

 is always opposite to the point where the sperm has penetrated. This point is 

 consequently medio-ventral. Moreover, it can be demonstrated that if fertilization 

 is experimentally localized, the spermatozoon may penetrate at any point either 

 in the polar or in the equatorial region, and the grey crescent will be just opposite 

 (Ancel and Vintemberger, 1948). In this case, bilateral symmetry is determined 

 by the experimenter, and may be said to be acquired by epigenesis (Fig. 14). 



However, the question is whether or not this conclusion holds for eggs fertilized 

 under normal conditions, i.e. when the female lays her egg in a pool. It is not 

 impossible that some part of the cortex would offer more favorable conditions 

 for the adhesion or incorporation of the spermatozoon, either by the chorion being 

 richer in fertilizin or being simply softer or thinner. No one has so far found a way 

 for testing this hypothesis. Pointing in favor of a non-random place of penetration 

 is the fact that indices of bilateral organization have been observed in the growing 

 oocyte (p. 316). The discrepancy existing, in this problem, between some anurans 

 and the other amphibians, suggests that the primary state of affairs is the existence 

 in the egg of a tendency to acquire its own, more or less preformed bilateral 

 organization, and that the epigenetic situation is a secondary acquisition of some 

 groups. 



Whatever the issue may be, the mechanism of symmetrization, in itself, has 

 been remarkably elucidated. The development of the spermatic aster is the primum 

 movens. Being somewhat eccentric, it repels the big yolk platelets of the sub- 

 equatorial region towards the opposite side of the egg. The yolk wall formed in 

 this way has, for some unknown reason, an effect on the cortical layer and the 

 hyaline pellicle. It produces in these layers a movement which begins dorsally, 

 continues polewards, proceeds ventralwards, and stops against the yolk mass 

 (Figs. 13-14). 



