352 ON THE NUMBER OF POLAR BODIES AND 



between the first and the second polar body. Butin many eggs it 

 appears certain that the second polar body is not expelled until the 

 spermatozoon has penetrated. O. Schultze, the latest observer of 

 the egg of the frog, in fact saw the first polar body alone extruded 

 from the unfertilized egg : a second nuclear spindle was indeed 

 formed, but the second polar body was not expelled until after 

 fertilization had taken place. A very obvious theory therefore sug- 

 gests itself: that while the formation of the second polar body is 

 purely a phenomenon of maturation in most animal eggs, and is 

 independent of fertilization, in the eggs of a number of other 

 animals, on the other hand, and especially among Arthropods, 

 the formation of the second nuclear spindle is the result of a 

 stimulus due to the entrance of a spermatozoon. If this sug- 

 gestion be confirmed, we should be able to understand why partheno- 

 genesis occurs in certain classes of animals wherever the external 

 conditions of life render its appearance advantageous, and further, 

 why in so many species of insects a sporadic parthenogenesis is ob- 

 served, viz. the parthenogenetic development of single eggs (Lepi- 

 doptera). Slight individual differences in the facility with which 

 the second nuclear spindle is formed independently of fertilization 

 would in such cases decide whether an egg is or is not capable of 

 parthenogenetic development. As soon, however, as the second 

 nuclear spindle is formed, parthenogenesis becomes impossible. 

 The nuclear spindle which gives rise to the second polar body, and 

 that which initiates segmentation, are two entirely different things, 

 and although they contain the same quantity, and the same kind 

 of germ-plasm, a transformation of the one into the other is 

 scarcely conceivable. This conclusion will be demonstrated in the 

 following part of the essay. 



II. THE SIGNIFICANCE OF THE. SECOND POLAR BODY. 



I have already discussed the physiological importance of the first 

 polar body, or rather of the first division undergone by the nucleus 

 of the egg, and I have explained it as the removal of ovogenetic 

 nuclear substance which has become superfluous and indeed in- 

 jurious after the maturation of the egg. I do not indeed know of 

 any other meaning which can be ascribed to this process, now that 

 we know of the occurrence of a first division of the nucleus in 



