February 2, 1906.] 



SCIENCE. 



189 



other members of the family Ectatommidse. 

 The mistakes he would then discover would 

 be, I realize, still more embarrassing than 

 those he has detected at so much longer range, 

 but the discussion of them might have a cor- 

 respondingly greater scientific value. And 

 yet it may be that even in this I am still beg- 

 ging the question or asking an unfair advan- 

 tage, for increasing knowledge often sets cruel 

 limits in the free fields of literary sport. 



O. F. Cook. 

 Victoria, Texas, 

 December l(i, 190.5. 



SPECIAL ARTICLES. 

 A NEW THEORY OF SEX-PRODUCTION.' 



The last volume of the Proceedings of the 

 German Zoological Society contains an inter- 

 esting address by Professor Eichard Hertwig 

 in which is developed a new theory of sex- 

 production based on his long-continued ex- 

 periments on protozoa and applied to the in- 

 terpretation of the results of new experiments 

 by himself on amphibia and those of his 

 pupils Issakowitsch and von Malsen on daph- 

 nids (Simocephalus) and on Dinophilus. 

 Professor Hertwig's conclusions demand espe- 

 cial attention, since they are on the whole 

 antagonistic to the view, which has been 

 widely accepted in recent years, that sex is 

 already determined in the fertilized egg, 

 though he does not deny that such early de- 

 termination may exist in some cases. 



The new experimental results brought for- 

 ward are as follows: The work of Issakow- 

 itsch (since published in full in the Biolo- 

 gisches C entralhlatt) proves that in Simo- 

 cephalus sex-production shows a definite reac- 

 tion to temperature changes. At 24° C. a 

 parthenogenetic production of females, with 

 only the occasional appearance of a male, con- 

 tinues until the culture dies out; while a 

 reduction to 16° quickly leads, and reduction 

 to 8° immediately leads, to the appearance 

 of male« and later (sometimes immediately) 

 to the production of winter eggs. Issakow- 

 itsch also shows that a similar effect may be 



' R. Hertwig, ' Ueber das Problem der sexuellen 

 Differenzierung,' in Verhandlungen der Deutschen 

 Zoologischen Gesellschaft, 1905. 



produced by starvation without change of 

 temperature, and hence he concludes (like 

 Nussbaum in the case of rotifers) that the 

 change of temperature probably acts indirectly 

 through its effect on nutrition. Von Mal- 

 sen's work on Dinophilus shows that in a cul- 

 ture maintained for months at 10°-12° C. 

 male and female eggs are produced in a ratio 

 of 1:3, while at 25° the ratio rises to 1 : 1.T5, 

 and sometimes reaches 1 : 1. Both these eases 

 seem free from the objection that applies to 

 so many of the earlier experiments on sex- 

 modification that the statistical results may 

 be vitiated by different rates of mortality in 

 larvae of different sexes. Unfortunately the 

 same can not be said of Professor Hertwig's 

 own experiments on frogs, ingenious and in- 

 teresting as they are. Experiments with 

 changes of temperature gave no really satis- 

 factory result, though Hertwig seems inclined 

 to believe that higher temperatures favor the 

 production of females. On the other hand, 

 either over-ripeness or under-ripeness of the 

 eggs (a condition obtained by artificially de- 

 laying or hastening fertilization) led in every 

 case to a large excess of males. Like those of 

 earlier observers, these results are not very 

 convincing, owing to the high mortality of the 

 larvae, which must be reared to the time of 

 metamorphosis before the sex can certainly be 

 determined. The most satisfactory results 

 appear to have been obtained from a culture 

 of over-ripe eggs in which 20 per cent, to 30 

 per cent, of the fertilized eggs were reared 

 to this period, the result being 317 males to 

 13 females; and in one case of under-ripe 

 eggs 40 larvae that were successfully reared 

 were all males. These results can hardly be 

 ascribed to accident; but the dubious charac- 

 ter of the statistical data obtained by rearing 

 tadpoles and other larvag through long periods 

 of time has been so clearly shown by the ex- 

 perience of many other observers that the true 

 interpretation of the facts in this cas^ seems 

 by no means clear. 



Hertwig's general theory of sex-production 

 was primarily suggested by his own earlier 

 experiments on the relation between nucleus 

 and protoplasm in the protozoa. These ex- 

 periments led him to the conclusion that the 



