202 PROBLEMS OF FERTILIZATION 



that in his first group the spermatozoa fail to penetrate 

 the egg membranes, and this is probably so, but no 

 experiments have been undertaken to attempt to bring 

 about fertilization by artificial means in these cases. 

 Bataillon found in the case of the eggs of Pelodytes 

 or of Bujo activated by the sperm of Triton that the 

 latter had not entered at all, but had at most pierced 

 the cortex of the egg; he was therefore led to inquire 

 if a similar piercing by a fine needle might not bring 

 about the same results, and in this experiment he suc- 

 ceeded in producing complete parthenogenesis, as is 

 generally known. 



It is obvious that the explanation of the curious 

 results in hybridization of Amphibia cannot be given 

 in terms of chemical specificity alone. Pflliger con- 

 cluded that in general those spermatozoa are most 

 successful in cross-fertilization that have the thinnest 

 heads and sharpest perforatorium; and that eggs are 

 most accessible to hybridization when the spermatozoa 

 of the same species have thicker heads. He had thus 

 a conception that was based purely on the old idea 

 of the mechanical penetration of the spermatozoon 

 into the egg. The results on the Amphibia do not 

 exclude a certain amount of chemical specificity. Fuller 

 knowledge of the mechanism in these forms is necessary 

 for an explanation of the results. 



Many of the hybrid fertilized eggs die in the blas- 

 tula stage, but some combinations at later stages. 

 This has usually been attributed to an incompatibility, 

 chemical or otherwise, between the hybrid chromatins. 

 This idea has been supported in an interesting way by 

 O. Hertwig (1913) and G. Hertwig (1913). The basis 



