558 P. W. WHITING 



to be of considerable importance in heredity. In hybrids the 

 development is often normal up to this time, apparently because 

 the chromosomes from the different species remain distinct. 

 But when the chromosomes try to unite in synapsis they are 

 unable to do so in a normal manner and greater or less sterility 

 results. This has been shown by Guyer ('00) in hybrid pigeons, 

 by Federly ('13) in moths of the genus Pygaera, and by Wodse- 

 dalek ('16) in the mule. 



As has been pointed out by Metz ('16) there is a union of the 

 homologues in Diptera before cleavage is completed. This may 

 account for the sharp delimitation of species and the non-occur- 

 rence of hybrids in this group, for, if matings of different incipi- 

 ent species took place, it is possible that the early cleavage cells 

 resulting might become pathological through failure of the chro- 

 mosomes of different origin to unite properly in pseudosynapsis. 

 Thus the species would be separated in their incipiency and tend 

 to diverge under the influence of this 'physiological isolation.' 

 In the Lepidoptera pseudosynapsis has never been observed, 

 and hybrids and species-intergrades are common, as appears 

 from taxonomic work and especially from the work of Harrison 

 and Doncaster ('14). 



It is probably during the synaptic condition in the first ga- 

 metocyte that factors brought in by homologous chromosomes 

 have an opportunity to 'cross-over.' The synaptic conditions, 

 in forms in which partial coupling occurs, have never been 

 investigated. In plants a number of cases of partial coupling- 

 occurs and it appears that this is partial in the formation of 

 both male and female gametes. Bridges ('14) has summarized 

 some of this data from Bateson and from Gregory and shows that 

 the linked factors of the sweet pea and of Primula may bear a 

 linear relationship to each other. Castle and Wright ('15) also 

 have shown that in the rat linkage is partial in both sexes. 

 Sturtevant ('15) has pointed out that in the silk-worm moth, 

 the experiments of Tanaka show that linkage is complete in 

 the female, but partial in the male. The extensive work on 

 Drosophila by Morgan and his students has shown that linkage 



