D. W. Cutler 159 



I am at present unable either to confirm or dispute these results as 

 I have not obtained figures which are sufficiently clear to make an 

 accurate count of the chromosomes possible. Spermatogonial mitoses 

 in which an accurate count can be made are exceedingly rare, but from 

 the few figures I have observed I should conclude that the number is 

 from 18 to 20. 



In the primary spermatocytes the chromosomes are arranged in a 

 ring with one element projecting beyond the others, as described by' 

 Guyer, and the number is from 8 to 10. 



The aggregation of the chromosomes into clumps during the growth 

 of the secondary spermatocytes is very obvious in the sections, but I have 

 not been able to find any trace of degenerating spermatids, though I 

 have made a special search for them. 



Spermatogenesis of Hybrids. 



A careful examination of the testes of the two colour classes of hybrid 

 birds failed to show any differences whatsoever between them : the two 

 types will therefore be treated under the same heading. The testicular 

 tubules do not differ from those of the parent species, nor is there any 

 increase in the amount of interstitial tissue as was found in the sterile 

 tortoiseshell tom cat investigated by Doncaster and myself 



The spermatogonial cells are arranged round the margin of the 

 tubules and many are found in the dividing condition, which is in sharp 

 contrast with that of the parent species where dividing spermatogonia 

 are rare. The divisions are normal, the chromosomes arrange themselves 

 in a regular way on the spindle and pass to each pole without any trace 

 of lagging (Figs. 11, 12, 13). When seen in side view the impression is 

 given that good countable figures would be obtained from a face view ; 

 this, however, is not the case for the chromosomes stick together, as in 

 the parent birds, rendering it impossible to make an accurate estimation 

 of the number. It is probable, however, that it is from 18 to 20 (Figs. 14, 

 15). It is not possible to distinguish between the maternal and paternal 

 chromosomes. 



The early development of the primary spermatocytes is normal; a 

 granular spireme is formed (Fig. 16) which retracts to one side of the 

 nucleus (Fig. 18), producing a closely packed mass of threads (Figs. 17, 

 19). The next stage is the one of interest in connection with the 

 problem of sterility for it is now that spermatogenesis becomes abnormal. 

 The synaptic threads, instead of breaking into bivalent chromosomes, 



