76 SCIENCE PROGRESS 



The comparative ease, therefore, with which the sex ratio 

 of the frog can be affected by external condition renders it 

 difficult to draw deductions as to the cause of dual sex-production 

 from parthenogenetic eggs. There is no doubt but that 

 important experimental evidence as to determination of sex 

 will result from such work, when more animals, obtained from 

 unfertilised eggs, have been raised to maturity, and their 

 cytology completely investigated. At present, all that can 

 be asserted is that the scanty data does not contradict the 

 assumption that the sex of an animal is, at least to a certain 

 extent, correlated with the presence or absence of a particular 

 chromosome or chromosomes. 



Before closing this article reference must be made to Her- 

 lant's most suggestive work as to the factors underlying artifi- 

 cial parthenogenesis. According to Loeb, it is first due to a 

 cytolytic action on the egg-surface causing nuclear division. 

 Such action, however, leads to disintegration unless checked 

 by a second agent — commonly hypertonic sea-water — inhibiting 

 excessive oxidation. The further assumption is then made 

 that at normal fertilisation the sperm introduces into the egg 

 a cytolytic agent, inducing cell-division ; and that an inhibiting 

 substance is also carried by the sperm. 



Delage, on the other hand, believes that cell-division is a 

 series of coagulations and liquefactions of the colloidal proto- 

 plasm, and that the agents used to cause artificial parthenogenesis 

 simply set in motion this series of coagulations and lique- 

 factions. One of the weak points of this theory is that such 

 cyclic changes have never been satisfactorily demonstrated — 

 a criticism which, to a less extent, applies to Loeb's speculations. 



A third theory, analogous to that of Delage in that no 

 postulation is made as to the introduction into the egg of 

 definite chemical substances, assumes that the mechanism of 

 division is latent in the egg and is set in action by substances 

 altering the permeability of the egg-membrane. Lillie therefore 

 believes that at fertilisation there is initiated altered conditions 

 of interchange of diffusible substances and ions, through 

 rhythmical changes in the permeability of the cell-membrane. 



These theories, and others not mentioned, are admittedly 

 highly speculative, and too purely physico-chemical to be 

 completely satisfactory. Herlant's work is therefore par- 

 ticularly interesting in giving a different view-point of the 

 problem, and, though his conclusions are no more completely 

 satisfactory than those of other workers, they are not less 

 important. 



Herlant maintains that all methods of inducing artificial 

 parthenogenesis are such that either the effective volume of 

 the cytoplasm is reduced or else the size of the division spindle 



