ARTIFICIAL PARTHENOGENESIS IN ANNELIDS 683 
In Podarke, an Annelid, I succeeded in producing the 
first segmentation in unfertilized eggs. I interrupted these 
experiments to go on with experiments on Cheetopterus 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 parthenogenetically. 
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, Chetopterus 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 Chetopterus, 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 
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