162 Hjihrldi^ between Plieasant and FoirJ 



male about 10 clinimnsomes paired, but in the converse cross the ininiber 

 was greater. 



The reduced number in zonarui is 56, while that of Idrtaria is 14. 

 It will be noticed that the number in the latter species is four times 

 that of the former, and Doncaster suggests that in liirtaria the chromo- 

 somes are compound and the soiuiria units are here united together, 

 but in a different order. If this is so the further suggestion is made 

 that sterility is due to the chromosomes not being able to pair normally 

 because of the different grouping of the imits. 



In connection with the foregoing, the results of Federley's work are 

 of interest. Pygaera curtula male and Pygaera anachoreta female were 

 mated and the offspring, which were fertile, showed certain characters 

 of one species dominant and othei- characters intermediate. On crossing 

 the hybrifls back to one of the parent species segregation of a few 

 characters occurred. 



Cytological investigation showed that during maturation of the 

 oi-iginal hybrids there was practically no pairing of the chromosomes; 

 one or two did mate, but at division they segregated, while the remainder 

 divided equationally, giving rise to apparently normal spermat(jcytes. 



In the gonads of a hybrid formed by mating a male of the original 

 hybrids back to a pure tnutchorehi female, there was a triple set of chromo- 

 somes composed of a double set of anachoreta and a single set of curtula. 

 Pairing of the anachoreta chromosomes occurred at maturation, while the 

 curtula ones divided equationally. The gametes produced, therefore, had 

 a haploid .setof (()(ac/(0/-eia. and curtula. chromosomes. 



The fertility of these hybrids may then be due to the almost entire 

 lack of pairing of the chromosomes in the first hybrids produced. 



Reviewing the cases enumerated above it is apparent that sterility 

 is primai'ily due to the incompatibility of the maternal and paternal 

 germ plasms: this is sometimes manifested by the inability of the chromo- 

 somes to pair and separate in a normal way during synapsis, while in 

 other cases the repellent nature of the germ plasms seems to be so great 

 that pairing occurs only to a slight degree. In other hybrids, however, 

 .synapsis takes place, but bivalent chromosomes are not formed, the 

 chromatin threads bi-eaking into irregular masses. 



It is evident, however, that whatever may be the cause of the 

 incompatibility of the chromosomes, it is not sufficient to prevent the 

 functioning of the rest of the cells of the body, which appear to perform 

 their several duties in a normal manner. Further we saw that in the 

 Pygiiera hybrids, which wrw (piitc fertile, there was little or no pairing 



