218 THE PHYSIOLOGY OF REPRODUCTION 



sea? water, they sometimes proceeded to segment. The latter 

 process, however, was not normal, since the ova that had been 

 subjected to this treatment became transformed into masses of 

 minute granules, and, instead of acquiring cilia and giving rise to 

 embryonic individuals, they underwent a process of disintegration. 



To Loeb belongs the credit of having done more than any 

 other worker to elucidate the physico-chemical aspects of the 

 phenomena of fertilisation. Loeb was the first definitely to 

 succeed in producing plutei from the unfertilised eggs of the 

 sea-urchin. His original method was to expose the eggs for 

 about two hours to sea-water in which the degree of concentra- 

 tion had been raised by about forty or fifty per cent. This 

 effect could be produced by the addition of sodium chloride, but 

 it was found to be immaterial what particular substance was 

 employed to raise the concentration, so long as it was one which 

 did not act injuriously on the eggs. The ova were afterwards 

 restored to normal sea-water, when they began to undergo 

 segmentation and subsequently developed into normal plutei. 



Loeb was able to show, further, that the parthenogenetic 

 development of the ova in such cases was brought about by a loss 

 of water. Thus, when the concentration of the sea-water was 

 less than forty per cent., some of the ova of the sea-urchin Arbacia 

 could be induced to develop, even though they were allowed to 

 remain in the hypertonic solution. By adopting similar methods 

 a like result could be effected for the other species of sea-urchin, 

 and also in the case of the starfish Asterias jorbesii ; but it was 

 necessary, as a general rule, to restore the ova to normal sea- 

 water, as the continuance of abnormal conditions, although it 

 might not hinder segmentation, usually arrested the further 

 course of development. 1 



It was found, however, that osmotic fertilisation differed 

 in several respects from fertilisation by a spermatozoon. 

 Firstly, the ova fertilised by the former method began to seg- 



1 Loeb (J.), "On the Nature of the Process of Fertilization," &c., Amer. 

 Jour, of Phys., vol. iii., 1899. "On the Artificial Production of Normal 

 Larvae from the Unfertilised Eggs of the Sea-Urchin (Arbacia)" Amer. Jour, of 

 Phys., vol. iii., 1900. " On Artificial Parthenogenesis in Sea-Urchins," Science, 

 vol. xi., 1900. " Further Experiments on Artificial Parthenogenesis," &c., 

 Armr. Jour, of Phys., vol. iv., 1900. These papers are reprinted in Loeb's 

 Studies in General Physiology, vol. ii., Chicago, 1905. 



