44 SCIENCE PROGRESS 



the case of the purely parthenogenetic gallflies, they appear 

 to be able to reproduce for an indefinite number of generations 

 without the intervention of a true sexual process. On the other 

 hand, a very large proportion of sawflies are always bisexual, 

 and their eggs never develop unfertilised. A disappearance of 

 the male and indefinite reproduction by parthenogenesis appears 

 to have occurred also in some species of Ostracoda among the 

 Crustacea, but it is very exceptional in the animal kingdom. 



The case of the sawflies, which produce males from un- 

 fertilised, females from fertilised eggs, leads on to that of the 

 honey-bee and others of the same class, in which all the eggs 

 appear to be alike, but when they are fertilised they give rise to 

 females, when they develop without fertilisation they yield 

 males. It appears to be established almost beyond possibility 

 of doubt that in many of the aculeate Hymenoptera, and also 

 some of the parasitic families, the sex is determined by fertilisa- 

 tion. And in some of the cases mentioned above, e.g. Daphnia, 

 Aphis, and Hydatina, although parthenogenetic eggs might give 

 rise to either males or females, yet, as in the bee, fertilised eggs 

 are all female, so that here again it is possible that fertilisation 

 determines the sex. But in the one case (the bee, etc.) the 

 eggs are all alike, and differ only in whether they are fertilised 

 or not, in the other some eggs require fertilisation if they are to 

 develop, others are not capable of it in any case. 



Artificial differs from natural parthenogenesis in the fact that 

 it does not occur without some external stimulus which the eggs 

 will not commonly experience in nature. But in some species, 

 e.g. certain moths, parthenogenesis occurs exceptionally in 

 nature, but can apparently be made more frequent by apply- 

 ing certain stimuli, such as shaking the eggs or treating them 

 for a very short time with acid. Also in the Echinoderms, the 

 group in which artificial pathogenesis has been most thoroughly 

 studied, some species are naturally parthenogenetic, and it would 

 seem that the stimulus supplied brings into action a tendency 

 which normally is not strong enough to take effect. 



The artificial means used for causing eggs to develop without 

 fertilisation are very varied ; it was first done by Loeb, by the 

 addition of magnesium salts to the water containing them, and 

 he supposed the magnesium exerted a specific influence. It soon, 

 however, appeared that other salts were effective, and ionisation 

 and osmotic pressure were suggested as causes. Since then 



