1 86 SCIENCE PROGRESS 



Is it not possible that the perplexing occurrence of vicarious 

 avicularia in some of, but not by any means in all, the colonies 

 of certain species may be interpreted as a reversion due to the 

 combination of two or more allelomorphs that may not have 

 occurred together in the parental forms ? 



The President's example was followed when the zoologists 

 and botanists met to inquire into recent work on the deter- 

 mination of sex. By far the most original contribution to the 

 debate was Mr. Doncaster's record of breeding from moths. 

 He began by sketching the early history of the question of 

 sex determination till it reached the stage where Wilson and 

 others discovered that in certain insects the males and the 

 females contain different numbers of chromosomes in the germ- 

 cells, the females having an even number, the males one less. 

 After maturation there are two kinds of spermatozoa, one 

 containing the same number as the mature egg, and the other 

 having one chromosome missing. It was at first suggested 

 that at fertilisation the spermatozoon having the larger number 

 caused the egg to develop into a female, that with the smaller 

 number male ; but Wilson's later suggestion is that there is 

 selective fertilisation, that the eggs are either male or female, 

 and that male eggs are fertilised by spermatozoa having no 

 heterochromosome, female eggs by those which have it. 

 Morgan has recently found that in a species of Phylloxera 

 there are two kinds of spermatids, one of which has one 

 chromosome more than the other. Those with the smaller 

 number degenerate ; those with the larger develop into 

 functional spermatozoa, and all fertilised eggs become females. 



One particular experiment illustrates this remarkably. In 

 the moth Abraxas Grossulariata is a rare variety lacticolor, 

 which is found usually only in the female. This acts as a 

 Mendelian recessive, and the tables of generations were ex- 

 tremely remarkable. From them Mr. Doncaster argued that 

 the sex determinants behave as Mendelian characters, maleness 

 and femaleness being allelomorphic with one another and 

 femaleness dominant. All females are heterozygotes, carrying 

 recessive maleness, and producing male-bearing and female- 

 bearing eggs in equal numbers ; all males are homozygous, 

 carrying only maleness and producing only male-bearing 

 spermatozoa. 



The history of the moth has a host of interesting Mendelian 



