VI.] THEIR SIGNIFICANCE IN HEREDITY. '>fi'>, 



development of single eggs (Lepidoptera). Slight individual 

 differences in the facility with which the second nuclear spindle 

 is formed independently of fertilization would in such cases 

 decide whether an o^gg 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 demon- 

 strated 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 ^gg^ and I have explained it as the removal of 

 ovogenetic nuclear substance which has become superfluous 

 and indeed injurious after the maturation of the ^gg. 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 parthenogenetic as well as in sexual 

 eggs. A part of the nucleus must thus be removed from both 

 kinds of eggs, a part which was necessary to complete their 

 growth, and which then became superfluous and at the same 

 time injurious. In this respect the observations of Bloch- 

 mann ^ upon the eggs of Miisca vornitoria seem to me to be very 

 interesting. Here the two successive divisions of the nuclear 

 spindle arising from the egg-nucleus take place, but true polar 

 bodies are not expelled, and the two nuclei corresponding to 

 them (one of which divides once more) are placed on the 

 surface of the ^gg, surrounded by an area free from yolk 

 granules ; and they break up at a later period. The essential 

 point is obviously to eliminate from the egg-cell the influence 

 of nucleoplasm which has been separated from the egg-nucleus 

 as the first polar body; and this condition is satisfied whether 

 the elimination is brought about by a process of true cell- 



^ 1. c. p. no. 



