ARTIFICIAL PARTHENOGENESIS 145 



all, and Professor Loeb 1 goes so far as to maintain that the 

 problem of fertilization is really one of physical chemistry. He 

 holds that the development of the egg is to be regarded as a 

 chemical process which depends mainly on oxidation, and finds 

 that the unfertilized eggs of various animals (sea-urchins and 

 worms) will undergo development (at any rate up to a certain 

 point) after exposure to the action of certain chemical reagents. 

 The unfertilized eggs of a sea-urchin, for example, developed into 

 larvae after being placed for two hours in sea water, the osmotic 

 pressure of which had been raised about 60 % by the addition of 

 some kind of salt or sugar, but this " hypertonic '' solution must 

 contain a sufficient quantity of free oxygen. In another case the 

 unfertilized eggs of the worm Chaetopterus were stimulated to 

 develop into larvae by the mere addition of potash and acids, 

 without the osmotic pressure of the sea water being raised. 



Exactly what takes place under these circumstances we do not 

 know, and any speculation on this point is perhaps somewhat 

 premature, but it is quite clear from the experiments of Loeb and 

 other workers in the same field that we can no longer regard 

 fertilization as an indispensable condition of development even in 

 the case of eggs which do not naturally exhibit the phenomenon 

 of parthenogenesis. These experiments may also throw some 

 light upon the process of normal fertilization, especially as 

 regards the nature of the actual stimulus which causes the 

 fertilized egg to begin segmenting. 



The casting out of polar bodies during the maturation of the 

 ovum led many of the earlier observers of this phenomenon to 

 believe that the matured ovum is incapable of development 

 because it has an imperfect nucleus, the importance of the 

 nucleus as taking the lead in cell-division having been established 

 at a comparatively early date. The imperfect nucleus of the 

 matured ovum was termed the female pronucleus, and it was 

 supposed to be converted into a perfect segmentation nucleus by 

 union with the male pronucleus brought into the egg by the 

 spermatozoon (Fig. 64), and the power of cell-division was supposed 

 to result from this completion of the nucleus. The observations 

 upon which this belief was based were perfectly correct, but the 

 conclusions drawn from them have not been sustained by recent 

 investigations, for in cases of artificial parthenogenesis development 



1 "Die chemische Entwicklungserregung des tierischeii Eies" (kiinstliche Par- 

 theuogeiiese). Berlin, 190'J. 



B. L 



