Page:The Zoologist, 4th series, vol 5 (1901).djvu/142

118 I., published last November by the "Millport Marine Biological Station," contain a summary of a paper "On Negative Evidence regarding Influence of Nutrition on Sex," by J.F. Gemmill, Lecturer on Embryology, University of Glasgow.

It is well known that during a certain period in the early development of most animals no difference, external or internal, can be detected between the males and females. This undifferentiated stage is considered by some to be a stage of sexual indifference more or less complete. There is a well-known theory that during the period in question the future sex of individuals may be influenced by nutrition in such a way that a female bias is given by rich and plentiful food, and a male bias by the opposite condition. To test this hypothesis, Mr. Gemmill selected two molluscs for observation—the Edible Mussel (Mytilus edulis), and the Common Limpet (Patella vulgata). We give some extracts as regards the evidence afforded by the first:—

"Mussels may be found in suitable places on the tidal zone at all levels' from a little below high-water mark, downwards. They feed only when immersed under water—that is to say, when the tide is sufficiently far in to cover them. Mussels placed high up on the tidal zone are thus able to feed only intermittently, and during short periods. They grow slowly, and remain small in size. The average weight of low-level Mussels on Keppel Pier, Millport, is six or seven times greater than the average of high-level specimens. These facts and similar facts regarding the fixed animals on the tidal zone (e.g. Barnacles) point to comparative starvation of high-level specimens as contrasted with those which occupy a lower position.

"In the life-history of the Mussel there is first a free swimming stage, then the larvæ settle down and become permanently attached. At the time when they fix themselves, and for a considerable period afterwards, their sex, so far as I could make out by careful histological examination, is still undifferentiated. During the first period of growth and differentiation of sex, the young Mussels are subject to the different nutritive conditions indicated above, according as they have placed themselves high up or low down on the tidal zone. Nature thus supplies all the factors necessary for an experiment, and works out the experiment herself. It only remains to observe, in a large number of specimens taken from different localities, the relative proportion of males and females at different levels. By comparing these proportions we shall be able to tell whether richness or poorness of nutrition, acting under the conditions above indicated, have had any influence in determining sex in the young Mussel. A large number of Mussels were accordingly examined. In order to avoid local peculiarities, the batch of Mussels were taken from many different localities.

"Results.—Out of the total number examined, 49 per cent. were males,