FOUNDATION OF A THEORY OF HEREDITY. 245 



to our present view, any more than the fact that an Infusorian may 

 die from starvation would be an objection to the supposition of the 

 immortality of unicellular organisms. The growth of an organism 

 is not only arrested by its constitution, but also by absolute want 

 of food ; but it would be very foolish to explain the differences 

 in size of the various species of animals as results of the different 

 conditions of nutrition to which they were subject. Just as 

 a sparrow, however highly nourished, could never attain the size or 

 form of an eagle, so a germ-cell destined to become a summer-egg 

 could never attain the size, form, or colour of a winter-egg. It is 

 by internal constitutional causes that the course of development is 

 determined in both these cases ; and in the latter, the cause can 

 hardly be anything more than the different constitution of the 

 nucleoplasms. 



All these considerations depend upon the supposition that the 

 egg-nucleus contains two kinds of idioplasm, viz. germ-plasm and 

 ovogenetic nucleoplasm. I have not hitherto brought forward any 

 direct evidence in favour of this assumption, but I believe that such 

 proofs can be obtained. 



It is well known that there are certain eggs in which the polar 

 bodies are not expelled until after the entrance of spermatozoa. 

 Brooks 1 has already made use of this fact as evidence against 

 Minot's and Balfour's theory ; for he quite rightly concludes that 

 if the polar bodies really possess the significance of male cells, we 

 cannot understand why such eggs are unable to develope without 

 fertilization, when they still possess the male half of the nucleus 

 necessary for development. But such eggs (e.g. that of the oyster) 

 do not develope, but always die if they remain unfertilized. 



This argument can only be met by a new hypothesis, the con- 

 struction of which I must leave to the defenders of the above- 

 mentioned theory. But the observation in question seems to me 

 to furnish at the same time a proof of the co-existence of two 

 different nucleoplasms in the egg-nucleus. If the nucleoplasm of 

 the polar bodies was also germ-plasm, we could not understand 

 why such eggs are unable to develope parthenogenetically, for at 

 least as much germ-plasm is contained in the unfertilized egg as 

 would have been present after fertilization. 



1 Brooks, 'The Law of Heredity.' Baltimore, 1883, p. 73. 



