History of the Theory of Heredity. 73 



partlieuogenetic eggs of Rhodites, and he suggests that 

 these may be the long-sought polar cells, but he does 

 not feel certain that this is the case, and examination of 

 his paper will show that there is so much difference be- 

 tween the early stages of insect eggs and the corre- 

 sponding stages of simpler and more typical eggs, that 

 the identity of these bodies must remain open to some 

 doubt, but there can be no doubt of the nature of the 

 polar cells described by Grobben in the parthenogenetic 

 eggs of Moina. 



There is another objection to the hypothesis, which 

 seems to me to be entitled to great weight. According 

 to Balfour's statement we should ex^^ect that any egg 

 which retained the polar cells might develop without 

 impregnation. Observers have failed to discover their 

 extrusion in the eggs of ordinary arthropods, as well as 

 in those which are parthenogenetic, and we should 

 therefore expect all the arthropods to be parthenoge- 

 netic, but this is not the case. In many other animals, 

 as in the oyster, they are not discharged until the egg 

 is fertilized, and the hypothesis would require us to 

 believe that an unfertilized oyster Qgg contains a male 

 element as well as a female element; but when perfectly 

 ripe oyster eggs are placed, without fertilization, under 

 conditions which are perfectly favorable to development: 

 they show no signs of life, and soon die and decay. If 

 a little male fluid is added, however, they quickly dis- 

 charge their polar cells, and then rapidly joass through 

 the changes which build up the embryo. 



If the polar cell is really equivalent to a male cell, 

 we certainly might expect these oyster eggs, which are 

 perfectly ripe, and, according to the hypothesis, con- 

 tain all that is necessary for development, to show 

 some power to develop without impregnation. If the 

 power to extrude polar cells ^^ has been acquired hy the 

 ovum for the express purpose of preventing parthenogen- 



