ANIMAL PARTHENOGENESIS 45 



similar effects have been brought about by change of tempera- 

 ture, and in the case of starfish eggs by anaesthetisation with 

 carbonic acid during the maturation process. 1 It is possible, 

 therefore, that the stimulus applied may be of various kinds ; 

 and what is required is to bring into activity a tendency to cell- 

 division and development which is not normally aroused except 

 by the entrance of the spermatozoon. 



Loeb, however, 2 considers that by far the most important 

 factor, and in most cases the only one, is the extraction of water 

 from the egg by osmotic tension. The eggs of some marine 

 worms have been caused to develop to larvae by the addition 

 of calcium or potassium ions without raising the osmotic tension ; 

 but for probably all other cases he regards the extraction of 

 water as the exciting cause. That it is not the subsequent 

 entrance of water when the eggs are put back into their normal 

 surroundings is shown by the fact that the eggs of some sea- 

 urchins develop in solutions of higher osmotic tension without 

 any return to normal conditions, and also because dilution of the 

 sea-water does not bring about development. 



These facts lead Loeb to an interesting comparison between 

 artificial fertilisation by hypertonic solutions, and normal fer- 

 tilisation by a spermatozoon. In echinoid eggs, one of the 

 most characteristic phenomena of fertilisation is the almost 

 instantaneous formation of a membrane around the egg when 

 the spermatozoon enters. Loeb shows that this is due to 

 expulsion of water from the egg, since if the process is watched 

 in eggs which have lain for many hours in sea-water, so that it 

 takes place slowly instead of rapidly, vesicles are seen to form 

 round the egg as the water is expelled, and these coalesce to 

 form the membrane. Unfertilised eggs may be induced to form 

 a similar membrane by putting them in rather strong solutions 

 of salts or sugar ; but such eggs generally break up rapidly 

 owing to the high osmotic tension. It seems, therefore, that 

 both in fertilised eggs and in those made to develop partheno- 

 genetically by artificial means, there is loss of water, and 

 it may be that this provides the stimulus which causes the 

 egg to segment. 



On the other hand, Kellogg 3 finds in the eggs of the silk- 



1 Delage, C.J?, cxxxv. Nos. 15, 16, 1902. 



3 Pfluger'sArch.f.ges. Phys. vol. ciii. 1904, p. 257. 



3 Biol. Bull. xiv. 1907, p. 15. 



