FOUNDATION OP A THEORY OF HEREDITY. 225 



III. ON THE NATURE OF PARTHENOGENESIS. 



It is well known that the formation of polar bodies has been 

 repeatedly connected with the sexuality of germ-cells, and that it has 

 been employed to explain the phenomena of parthenogenesis. I 

 may now, perhaps, be allowed to develope the views as to the 

 nature of parthenogenesis at which I have arrived under the in- 

 fluence of my explanation of polar bodies. 



The theory of parthenogenesis adopted by Minot and Balfour is 

 distinguished by its simplicity and clearness, among all other in- 

 terpretations which had been hitherto offered. Indeed, their ex- 

 planation follows naturally and almost as a matter of course, if the 

 assumption made by these observers be correct, that the polar 

 body is the male part of .the hermaphrodite egg-cell. An egg 

 which has lost its male part cannot develope into an embryo until 

 it has received a new male part in fertilization. On the other 

 hand, an egg which does not expel its male part may develope with- 

 out fertilization, and thus we are led to the obvious conclusion that 

 parthenogenesis is based upon the non-expulsion of polar bodies. 

 Balfour distinctly states ' that the function of forming polar cells 

 has been acquired by the ovum for the express purpose of prevent- 

 ing parthenogenesis 1 .' 



It is obvious that I cannot share this opinion, for I regard the 

 expulsion of polar bodies as merely the removal of the ovogenetic 

 nucleoplasm, on which depended the development of the specific 

 histological structure of the egg-cell. I must assume that the 

 phenomena of maturation in the parthenogenetic egg and in the 

 sexual egg are precisely identical, and that in both, the ovogenetic 

 nucleoplasm must in some way be removed before embryonic de- 

 velopment can begin. 



Unfortunately the actual proof of this assumption is not so com- 

 plete as might be desired. In the first place, we are as yet uncer- 

 tain whether polar bodies are or are not expelled by parthenogenetic 

 eggs 2 ; for in no single instance has such expulsion been established 

 beyond doubt. It is true that this deficiency does not afford any 



1 F. M. Balfour, ' Comparative Embryology,' vol. i. p. 63. 



2 The formation of a polar body in parthenogenetic eggs has now been proved : see 

 note at the end of this Essay; see also Essay VI. A. W., 1888. 



