142 



The Nucleus and Cytoplasm in Development 



culosa $) to normal Triturus embryos has 

 shown that grafted hybrid tissues may sur- 

 vive indefinitely and take part normally in 

 the formation of a variety of organs. It is 

 probable that the "lethal" effect of the Sala- 

 mandra chromosomes which expresses itself 



in the blastula is overcome or neutralized in 

 some unknown way by the surrounding tis- 

 sues of the normal host embryo (Luethi, '38; 

 Baltzer, '40). (See Baltzer, '52, for an alterna- 

 tive explanation involving "partial lethality" 

 of the hybrid tissues.) 



Fig. 36. Cytology of fertilization in crosses between different species of frogs and toads (after Tchou-Su. 

 '31). 



a, Rana 5 X Hyla ^ . Spermatozoon activates the egg on contact with egg surface but fails to penetrate. 

 Second polar body is given off, new egg aster appears but is unable to divide; one or more monaster cycles 

 multiply the chromosomes but do not induce cleavage. 



b, Hyla $ X Pelobates $ . Sperm penetrates, sperm aster associates with egg nucleus to produce a bipolar, 

 haploid mitotic figure. Sperm head fails to develop into a nucleus and degenerates; gynogenetic development 

 to haploid tadpole ("false hybrid"). 



c, Hyla $ X Bufo A, . Sperm head transforms into small sperm nucleus which fuses with egg nucleus; 

 sperm chromosomes are eliminated during telophase of first cleavage mitosis; gynogenesis as in b. 



d, Normal fertilization and cleavage; no elimination of paternal chromosomes, d', Bufo o x Rana 5 . 

 Hybrid dies in gastrula stage because of incompatibility between sperm chromosomes and egg cytoplasm. 

 d", Bufo vulgaris o x B. viridis $ . Normal development to viable diploid species hybrid. 



