vi HYBRIDS 185 



daughter nuclei and some being left behind and failing to enter either 

 nucleus. 



The nature of the progeny obtained by breeding from this hybrid 

 has already been described (p. 146). 



Somewhat intermediate between the two types which we have 

 provisionally distinguished as mechanical and physiological, stand 

 probably the Lepidopteran crosses examined by Federley (1913) and by 

 Harrison and Doncaster (1914). 



Federley's crosses (Fig. 79) were between Pygaera curtula (zn = 58) 

 and P. anachoreta (2n = 6o). The hybrid has about 29 + 30=59 chromo- 

 somes. In meiosis of the male hybrid very little syndesis takes place, 

 only two or three chromosomes being paired, so that there results a few 

 bivalents while the remainder are univalent. The former separate into 

 their constituents in the usual way, while the others divide as in a 

 somatic mitosis. Thus each gamete has the diploid number of chromo- 

 somes, except for those few which paired at syndesis, in respect of which 

 it is haploid. 



This hybrid was crossed back with anachoreta ? . The number of 

 chromosomes to be expected in this secondary hybrid is 59 +30 = 89 ; i.e. 

 a double set of anachoreta and a single set of curtula chromosomes or 

 nearly so, as the hybrid gametes have not quite the full 59 chromosomes. 

 This of course is a very high number to count satisfactorily, but in 

 several nuclei over 70 were counted. In meiosis of this secondary hybrid 

 we find a mixture of bivalents and univalents, leading to the presumption 

 that the anachoreta chromosomes introduced by the mother have paired 

 with those of the same species introduced by the hybrid gamete, while 

 the curtula chromosomes of the hybrid are left univalent. 



Harrison and Doncaster's cross was between Lycia hirtaria and 

 Ithysia zonaria, and gave results closely comparable to those of Federley 

 (Fig. 80). This cross exhibits an advantage over the last one described, 

 in that the chromosomes of the two parent species are distinguishable 

 from one another in the hybrid by their relative sizes. 



The chromosomes of L. hirtaria (2^ = 28) consist of n pairs of large, 

 i pair of small, and 2 pairs of very small ones. Those of /. zonaria 

 (2n = 112) are all very small, the largest being no bigger than the smallest 

 of L. hirtaria. 



In the hybrid diploid cells (spermatogonia) the two types of chromo- 

 somes are easily recognizable, the total number being of course 14 + 56 = 70. 

 In the first meiotic division it is found that there can be counted rather 

 less than 70, but always many more than 35 (varying from about 53 to 63). 

 It is therefore to be presumed that about a dozen pairs of chromosomes 

 have entered into syndesis, and that the rest remain unpaired. 



These hybrids are completely sterile. 



