ARTIFICIAL PARTHENOGENESIS IN ANNELIDS 683 



In Poclarke, an Annelid, I succeeded in producing the 

 first segmentation in unfertilized eggs. I interrupted these 

 experiments to go on with experiments on Chsetopterus which 

 were much more promising. 



IX. NATURAL AND ARTIFICIAL PARTHENOGENESIS 



In a definite although very small number of animals each 

 egg possesses the quality to develop partheriogenetically. 

 Instances of this are to be found in the bees, social wasps, 

 Bombyx, Psyche, Daphnia, plant lice and others. In all 

 these animals the egg can be fertilized also by a spermato- 

 zoon. How does it happen that in these forms, although 

 fertilization may occur, the egg is, under certain conditions 

 at least, able to develop parthenogenetically ? Our experi- 

 ments show, that if the constitution of the sea-water were 

 only slightly different, that is, if it contained a little more 

 K, Cha3topterus would have to be added to the list of nor- 

 mally parthenogenetic animals. What I stated in my pre- 

 liminary report is certainly true for Chsetopterus, namely, 

 that it is the constitution of the sea-water which prevents 

 many or certain forms from being "naturally" parthenoge- 

 netic. By reversing this statement we may say that in the 

 naturally parthenogenetic animals it may be due to the con- 

 stitution of the blood (or the sea-water?) that the egg can 

 develop without fertilization. 



The bridge between the phenomena of natural and artifi- 

 cial parthenogenesis is formed by those animals in which 

 physical factors decide whether or not their eggs develop 

 parthenogenetically. In plant lice parthenogenesis is the 

 rule only as long as the temperature is high or the plant has 

 plenty of water. If we lower the temperature or let the 

 plant dry out, sexual reproduction occurs. The drying out 

 of the plant causes the tissues of the lice to lose water. The 

 factor, loss of water, makes the artificial parthenogenesis of 



