ARTIFICIAL PARTHENOGENESIS 189 



dividuals that develop from parthenogenetic eggs are as 

 vigorous as those from eggs that have been fertilized. 

 We have seen that such eggs without being fertilized 

 are capable of producing sexual females and males. 

 In one case, at least, we have seen how the process is 

 accomplished. 



When we review the facts of natural parthenogenesis, 

 we find certain relations that arrest our attention. 



Most parthenogenetic eggs give off only a single 

 polar body, while fertilized eggs without exception give 

 off two polar bodies. This difference is clearly con- 

 nected with the fact that in parthenogenetic eggs the 

 full number or diploid number of chromosomes is re- 

 tained by the egg. 1 In fertilized eggs half the chromo- 

 somes are thrown out in one of the two polar bodies. 

 The number is made good by the chromosomes brought 

 in by the spermatozoon. 



But this difference does not in the least explain nat- 

 ural parthenogenesis ; for we have experimental evi- 

 dence to show, that an egg will develop when only half 

 the number of chromosomes is present — one set will 

 suffice. 



There is another fact about parthenogenetic eggs 

 that has, I believe, been generally overlooked. Many 

 of these eggs begin to develop into an embryo before 

 they reach the full size of the fertilized eggs of the 

 same species. This is true at least of the eggs of aphids, 

 phylloxerans, daphnians, and rotifers. I interpret this 



1 According to my observations on aphids and phylloxerans, the 

 synapsis stage is omitted in parthenogenetic eggs, hence there is 

 no union (or reduction) of the chromosomes. The omission of this 

 stage may have something to do with parthenogenesis, although it 

 is not evident what the relation may be. 



