TIUNSACTIONS OK SECTION D. 739 



Recently important evidence has been obtained from breeding experiments with 

 Lepidoptera, fowls, &c. In tbe moth Abraxas grossulariata there is a rare variety, 

 lacticolor, which is found usually only in the female. It is a Mendelian recessive, 

 80 that when paired with a typical male all the offspring are typical grossu- 

 lariata. The various possible matings give the following results: — 



Lact. ? X gross. (J gives cj gross., ? gross. 



Heterozygous ? x heterozygous <? gives <? gross., 9 gross., ? lact. 



Lact. ? X heterozygous <? gives <? gross., <J lact., $ gross., $ lact. 



Heterozygous ? x lact. $ gives J gross., 9. lact. 



Lact. 9 X lact. $ gives $ lact., ? lact. 



Wild gross. $ x lact. <? gives $ gross., ? lact. 



There are here two results of great importance — first, that lact. g can only 

 be produced from the mating lact. 5 x heterozygous ,f , for the converse mating 

 (heterozygous 5 x lact. ^) gives all ^s gross., a\\ ^ s lact. ; and, second, that 

 although /«c<. j y. -wild gross. $ gives all ofl'spring ^ro«s. of both sexes, yet the con- 

 verse cross wild gross. % x lact. $ gives all ^s gross., all Js lact. This last 

 result proves that wild and apparently pure gross J s are really heterozygous in 

 respect of the lacticolor character. 



The explanation appears to be as follows : The sex determinants behave as 

 Mendelian characters, maleness and femaleness being allelomorphic with one another 

 and femaleness dominant. All females are heterozygotes, carrying recessive male- 

 ness, and producing male-bearing and female-bearing eggs in equal numbers; all 

 males are homozygous, carrying only maleness and producing only male-bearing 

 spermatozoa. Further, the grossulariata character cannot be borne by a female- 

 bearing gamete. Then if G stands for grossulariata, L lacticolor, $ male deter- 

 minant, and 5 female determinant, we shall get the following results : — 



Mating. Constitution. Gametes. Offspring, 



r Female lact. . . .LL? S ■ . .L9,L<?"lL?G<? =gross. female. 

 \ Male gross. . . .GG<?<J. . . G S,G S jL S G $ =gross. male. 



Female lact. . . . hh f S . . .L9,L<?li^?^ ^' =.^^'^^s. female 



Male heterozygote . GL <? <? . . ■ G .? , L ^J; ? J | rX^.S." 



L c? L (J = lact. male. 



r Female gross. . .GL?(7. . .L$,Gt?lL?L^= lact. female. 

 [ Male, lact. . . .LL^<?. . .LJ',Ld'jGr(?L^ =gross, male. 



Wo can prove experimentally that all grossulariata females in this case are 

 heterozygous in respect of the lacticolor character, and that normal wild males 

 are homozygous and do not bear lacticolor, so that the variety can never appear 

 under ordinary circumstances. The simplest explanation that has been offered 

 for this fact is the one suggested, that females are also heterozygous in respect 

 of sex-determinants, while males are homozygous, and that in gametogenesis 

 there is repulsion between the female determinant and the grossulariata 

 determinant, so that the two cannot be borne by the same gamete. 



The following Papers were then read : — 

 1. The Early Development of the Marsupialia. By Professor J. P. Hill. 



2. The Gastrulation and Formation of Layers in Amphioxus. 

 By Professor E. W, MacBride, F.R.8. 



Two main types of theory as to the method in which the germinal layers 

 are formed in Amphioxus have been advanced. These are (1) That the invagina- 

 tion which changes the blastula into a gastrula is a simple process, and that the 



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