SECTIONAL TRANSACTIONS.—D. 397 
4. Prof. W. M. TarrprsAuu and Miss EK. M. Suerrarp.—Sezx Phases 
in the Female of Asellus aquaticus (Linn.). 
In the course of an examination of the changes which take place during the 
breeding season in the female of Asellus aquaticus, it was found that adults of 
this sex occur in one of two forms (1) a non-breeding form, in which the ooste- 
gites are present as small rectangular plates, not overlapping those in front or 
behind, and (2) a breeding form, in which the oostegites are large, foliaceous 
plates overlapping those in front and behind and those of the other side to form a 
functional brood pouch. These phases succeed one another rhythmically during 
_the breeding season, a breeding phase being regularly followed by a non-breeding 
phase and that by a breeding phase again, so that there are two moults between 
successive broods. ‘This series of events is quite different from that which 
Mrs. Sexton has found in Gammarus. There the oostegites, once fully 
formed, appear completely developed and functional at each moult, and there 
is no non-breeding phase between the egg-bearing phases. Once adult condi- 
tions are reached each stage of moulting is a breeding egg-bearing stage. 
Evidence supplied by an examination of a collection of Phreatoicus from 
_ Australia suggests that the same conditions obtain there as in Asedlus—i.e. there 
is an alternation of breeding and non-breeding phases during the breeding season, 
distinguished by the difference in the size of the oostegites. Further evidence 
is being sought to determine how far the differences between Gammarus and 
_ Asellus are characteristic of the Amphipoda and Isopoda as a whole. Uncon- 
firmed observations suggest that Asellus winters in the non-breeding phase. The 
relation of this phenomenon in Asellus to the phenomenon of high and low 
dimorphism known among the males of other Crustacea is discussed. 
5. Mr. J. S. Huxuny.—Linkage in Gammarus chevreuai. 
1. Sexual difference in linkage intensity.—The crossover value between the 
_ factors B and © in the Amphipod Gammarus chevreuxi is markedly different in 
the two sexes. In the female it is close to 50 per cent., while in the male it 
is about 20 per cent. Such a sexual difference in C.O.V. has only been found 
_ elsewhere in Orthoptera (Paratettiz Nabours). 
! This is of some theoretical interest, as it indicates that in Crustacea the male 
Sex is probably heterogametic, for in all animals so far investigated the hetero- 
_gametic sex shows stronger linkage-intensity than the monogametic—e.g. in 
Drosophila there is no crossing-over in the male, in the silkworm none in the 
female. In rats and mice there is crossing-over in both sexes, but slightly less 
‘in the male. The data from monecious plants do not bear upon the problem. 
2. Data are presented as to variations of linkage-intensity and their in- 
heritance. 
3. The possibility of the mechanism of linkage in Gammarus and some other 
forms being different from that in Drosophila is discussed. 
: 
6. Dr. F. A. E. Crew.—Studies in the Sexual Differentiation of the 
; Fowl. 
Bi 
» An account of certain experimental work which suggests that :— 
The type of differentiation of the embryonic gonad of the fowl is determined by 
the type of internal environment in which it finds itself, the type of internal environ- 
ment being determined by the genetic constitution of the individual. In an internal 
environment of ‘femaleness’ the embryonic gonad becomes an ovary, in one of 
-‘maleness’ a testis. The bird has an ovary because she is a female, a testis because 
he is a male. 
The type of plumage is determined by the type of metabolism which obtains at 
the time of its development, and is not a response to any specific influence of an 
‘internal secretion elaborated by the differentiated gonad. 
_ Size, shape, and carriage are characters that are not influenced by the type of 
gonad, as is clearly shown in the case of the developmental capon and poularde. 
> 
7. — ‘Reports of Committees. 
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