86 A. FRANKLIN SHULL 



NUCLEUS AND CYTOPLASM IN HEREDITY 



Unequal reciprocal hybrids are not uncommon both in animals 

 and in plants. Some of them are due to sex-limited inheritance, 

 and have received an explanation that is at least plausible, involv- 

 ing the accessory chromosome. In Oenothera the reciprocals 

 may resemble the mother most in some crosses, and resemble 

 the father^ most in other crosses; or may be more like the father 

 in some characters, more like the mother in others; these cases 

 must have a special explanation. In some cases, as echinoderms 

 (Baltzer '10), the inequality seems to be due to the elimination 

 of chromosomes in one of the reciprocals. But there remain a 

 number of cases which apparently belong to none of these cate- 

 gories. A few of these have been collected and discussed by 

 Jones ('12), who himself reports unequal reciprocals in Digitalis. 

 Pearl ('10) obtained a case in fowls very similar to that which 

 I here report in Hydatina; the 'hatching quality' of the eggs 

 was unequal in the two reciprocals.^ He suggested that the 

 greater infertility of the one reciprocal might be due to unfa- 

 vorable action of the secretions of the oviduct on the sperm in 

 that particular cross, but obviously unfavorable action by the 

 egg itself would bring about the same result. Toyama ('12) 

 has obtained unequal reciprocals in the silkworm. 



All cases in which the reciprocals are nearer the female par- 

 ent, unless having some other obvious explanation, might be used 

 as evidence that the cytoplasm is a partial bearer of the heredi- 

 tary qualities, and some of the cases found have been so inter- 

 preted. Certain echinoderm crosses, not necessarily reciprocals, 

 in which the zygote possessed purely maternal larval characters, 

 have received the same explanation. Unfortunately, in some of 

 the most striking cases, the hybrids could not be bred further, 

 owing to infertility, or to the fact that they never reached matur- 

 ity. Hydatina is particularly valuable in this regard, because 



-Since this paper went to press Professor Goldschmidt has explained certain 

 patrocline Oenothera hybrids as due to the degeneration of the maternal 

 chromosomes in the fertilized ovum. 



^ Dr. Pearl states that the inequality of the reciprocal hybrids was diminished 

 in the next generation. 



