186 SECTIONAL ADDRESSES. 



follows a uormal course the embryos are intermediate in regard to 

 character of the skeleton, thus affording proof of the influence of the 

 male parent. Another type of case is found in the .silkworm. Here 

 a certain rate character determining the time of hatching out of the 

 eggs has been shown to exhibit normal Mendelian inheritance, the 

 appearance that it is transmissible by the female through the cytoplasm 

 alone being delusive. The eggs are always laid in the spring. Accord- 

 ing as they hatch out immediately so that a second brood is obtained 

 in the year, or do not hatch out for twelve months, the female parent 

 laying the eggs is described as bivoltin or univoltin. Now the length 

 of interval before hatching is obviously an egg character, and therefore 

 maternal in origin. Consequently when a cross is made between a 

 univoltin female and a bivoltin male the eggs laid are not cross-bred 

 in respect of this character, any more than the seed formed as a result 

 of a cross is cross-bred in respect of its seed coat, which is a maternal 

 structure. The silkworm mother being univoltin, the eggs will not 

 hatch out until the following spring. The F^ mother will in turn 

 lay eggs which again take twelve months to hatch, since the long- 

 period factor is the dominant. It is not until the eggs of the F: 

 generation are laid that we see the expression of the character introduced 

 by the bivoltin father. For some of the egg batches hatch at once, 

 others not for twelve months, showing that of the F: females some 

 were uni- and some bi-voltin, and hence that the egg character in any 

 generation depends upon both the maternal and the paternal antecedents 

 of the female producing the eggs. Consequently, in the case of an 

 egg character the effects of inheritance must be looked for in the genera- 

 tion succeeding that in which the somatic characteristics of the zygote 

 become revealed. We find in fact that in almost all instances where 

 the evidence is suggestive of purely cytoplasmic inheritance, fuller 

 investigation has shown that the explanation is to be found in one of 

 the causes here indicated. The case of some plants where it has been 

 established that reciprocal hybrids are dissimilar still, however, remains 

 to be cleared up. Among such may be cited certain Digitalis hybrids. 

 Differences in the reciprocal hybrids of D. grandiflora and D. lutea 

 were described by Gaertner, and in the earlier literature dealing with 

 Digitalis species hybrids other cases are to be found. In more recent 

 years J. H. Wilson ^^ has I'epeated the crossing of D. purpureat and 

 D. lutea, and states that the reciprocals are indistinguishable during 

 the vegetative period, but that they differ in size and colouring of the 

 flowers, the resemblance being the greater in each case to the seed 

 parent. A detailed comparison of the differential characters of the 

 reciprocal hybrids of D. purpurea and D. grandiflora has been set out 

 by Neilson Jones, -^ who similarly finds in both matings a greater 

 resemblance to the mother species. We know notTiing as yet of the 

 cytology of these cases, and it is not improbable that the interpretation 

 may be found in some aberrant behaviour of the chromosomes. An 

 instance in a plant type where a definite connection appears traceable 

 between chromosome behaviour and somatic appearance has been 



2' Bep. Third International Congress on Genetics, R.H.S. 1906. 

 -- J. of Genetics, vol. ii., 1912. 



