SUSCEPTIBILITY OF AMPHIBIAN OVA TO X-RAYS 449 



beyond the cleavage stages but sooner or later, as a rule, becomes 

 abnormal. Occasionally adjustments take place of such a nature 

 that development may proceed at least to metamorphosis. Such 

 adjustments are most frequent when the paternal determinants 

 come from species closely related to the maternal, or when the 

 injurj^ to the paternal determinants has been slight. 



In crossing Bufo calamita and Pelodytes puntatus, Bataillon 

 found that apparently in the first division one cell received chro- 

 mosomes from the female pronucleus and the male pronucleus, 

 the other merely from the male pronucleus. In considering the 

 fertilization of amphibian eggs with exposed spermatozoa we 

 have seen reason to believe that at times the injured paternal 

 determinants become localized in certain regions so as to lead 

 to regional defects. Of these the most extreme and rarest exam- 

 ples are hemi-embryos. Unilateral defects in parts of the central 

 nervous system are far more common. 



In fertilized eggs of Nereis liillie found that by centrifugaliza- 

 tion he could destroy the activity of the male pronucleus, although 

 as a merely chemical complex it remained within the egg. In 

 those eggs which had formed the fertilization membrane and had 

 started development, differentiation proceeded to the formation 

 of the second polar body but no complete cleavage spindle was 

 formed and the eggs remained unsegmented. Here we must 

 assume the female pronucleus incapable of alone initiating cleav- 

 age unless we assume that it, too, was injured although less 

 obviously than the male pronucleus. 



In an interesting study of heredity in fundulus hybrids H. H. 

 Newman, ('10) finds that the developmental rhythum of the 

 young embryo is distinctly influenced by the foreign spermatozoon 

 as early as the second cleavage stage and comes to the conclusion 

 that the nuclear material is the chief factor in determining the 

 character of early development. Fischel has shown that in a 

 number of Echinoderm hybrids the male influence is expressed 

 structurally in the early blastula stages, not only in the general 

 size of the embryos, but also in the size and shape of the cells and 

 has given evidence that 'Hhe role of the spermatozoon is from the 

 beginning formative in character in that it is able to place the 



