The President's Address. By J. Arthur Thomson. 157 



and an unusual number of female offspring. The sperm may, as 

 it were, corroborate the bias of the ovum, for the percentage of 

 female offspring is higher when both parents are fed with lecithin. 

 It is not possible to follow the ova and prove that a relatively 

 anabolic one always becomes a female, and never a male, and so 

 on, but the argument from altered proportions seems sound. It is 

 interesting to notice Russo's statement that in nature male 

 offspring are always in the majority, from 52 to 58 per cent. A 

 curious result of Russo's experiments must also be mentioned, 

 though we cannot follow it up — that the more highly nourished ova 

 produce not only females, but females with the maternal colour. 



It has been objected to Russo's work that one of the two kinds 

 of ova he distinguished was due to degenerative changes, and that 

 he worked with selected families of rabbits. 



An acute friend, Dr. Ronald Macfie, has suggested to me that 

 it would be extremely interesting to try on both parents the 

 converse of Russo's experiment, by seeking to promote relatively 

 greater katabolism — for instance, by increased oxygenation. 



35. We should be slow, I think, to reject the view that changes 

 in nutrition and other environmental conditions may affect the 

 mother so as to alter the ordinary proportions of the sexes. 

 Issakowitsch, working with the parthenogenetic females of the 

 Daphnid Simocephalus, von Malsen, working with Dinophilus 

 apatris, in which the ova are fertilized, found that differences of 

 temperature affected the proportion of the sexes, apparently by 

 affecting the nutrition of the mothers. Both sets of experiments 

 are the more satisfactory in that they seem to be free from any 

 fallacy due to differential death-rate in the young of the two sexes. 



36. Many experiments have been made with the Rotifer 

 Hydatina senta, but the results are conflicting. There is great sex 

 dimorphism, the males being small and gutless. The females are 

 from birth either male-producers or female-producers ; and, accord- 

 ing to Maupas and ISTussbaum, this is determined before birth, 

 while the female embryo is still within its mother's uterus, by 

 conditions of temperature and nutrition. "Well-fed mothers produce 

 females which produce females only ; starved mothers produce 

 females which produce males only. According to I'unnett's 

 researches, however, changes of temperature and nutrition have no 

 effect ; but some stocks give rise to many male-producing females, 

 others to few or none. 



37. Another piece of evidence in support of the conclusion that 

 environmental influences may count is furnished by Nussbaum's 

 elaborate experiments on Hydra grisea, which he subjected to vary- 

 ing nutritive conditions. In this species hermaphrodite and dioecious 

 forms occur and periods of asexual budding. Nussbaum found that 

 he could bring on sexual reproduction by altering the nutrition, 

 that the optimum nutritive conditions were marked by a pre- 



April 19th, 1911 m 



