240 Hybrids of BiHtonine Moths 



typically in zonaria, seems not to occur (figs. 20, 21). In the mature 

 spermatocyte there are either one or two chromatin-nucleoli which are 

 clearly compound; the parts show less tendency to become separate 

 than in either of the pure species. 



Federley, in his work on hybrids between species of Fygaera, found 

 no synapsis (synizesis) stage in the hybrids ; in the present case, 

 although most of the chromosomes fail to pair, so that there is almost 

 the diploid number in the spermatocyte divisions, there is no important 

 difference between the spermatocytes in the earliest growth stage of 

 the hybrids and those of the parent species. The fact, however, that I 

 have found no "bouquet stage" with thick thread still contracted to 

 one side of the nucleus, is probably to be correlated with the fact that 

 most of the chromosomes fail to pair. Federley's failure to find any 

 synapsis (synizesis) in Pygaera hybrids may possibly be due to his 

 material being too old. In the present case I find such a stage very 

 frequently in larval testes, but not in pupal testes about a month older, 

 in which the majority of the follicles contain advanced spermatocytes, 

 and the earlier stages are scarce. The various stages seem to overlap 

 less in the hybrids than in the pure species. 



The spermatocyte division figures are very remarkable, and are not 

 conspicuously different from those of the spermatogonia. I have a 

 number of very perfect figures, and in all it is quite clear that the 

 chromosomes are nearly in the somatic number (figs. 22, 23), as was 

 found by Federley in his hybrids with Pygaera spp. Careful counts 

 show, however, that the full somatic number is not present, and that 

 some pairing of chromosomes has taken place. It is not easy to draw 

 the line quite clearly between the large and small chromosomes, for as 

 was said above the larger zonaria are similar in size to some of the 

 smaller of hirtaria. In the first spermatocyte equatorial plate there are 

 always about 12 or 13 which are certainly larger than all the rest, and 

 most of these may be regarded as hirtaria chromosomes. The number 

 of small ones is commonly about 50 ; careful counts have given 50, 50 or 

 51, and 51 or 52, in the three best figures I can find. Another fair 

 figure (slightly oblique, fig. 23) in which I cannot find that any chromo- 

 somes are omitted from the section, gives 13 large and about 40 small, 

 and others have given intermediate numbers ; it is possible that some 

 are covered by others, but it is unlikely that so many would disappear 

 in this way, and probably the smaller number may be due to the fact 

 that more chromosomes find mates in some cases than in others. There 

 seems no doubt that the total does not amount to the theoretical number 



