342 ON THE NUMBKK OF 1'OLAK BODIES AND 



germ-plasm, for it is capable of developing- into an embryo. We 

 have therefore, as it were, two natures in a single cell, which 

 become manifest one after the other, and which, according to our 

 fundamental conception, can only be explained by the presence of 

 two different idioplasms, which control the egg-cell one after the 

 other, and determine its processes of development. At first a 

 nucleoplasm leading to histological specialization directs the de- 

 velopment of the egg and stamps upon it a specific histological 

 character ; and then germ-plasm takes its place, and compels the 

 egg to undergo development into an embryo. If then the histo- 

 genetic or ovogsnetic nucleoplasm of the egg-cell can be derived 

 from the germ-plasm, but cannot be re-transformed into it (for the 

 specialized can be derived from the generalized, but not the 

 generalized from the specialized), we are driven to the conclusion 

 that the germ-plasm, which is already present in the youngest 

 egg-cell, first of all originates a specific histogenetic or ovogenetic 

 nucleoplasm which controls the egg-cell up to the point at which 

 it becomes mature ; that its place is then taken by the rest of the 

 unchanged nucleoplasm (germ-plasm), which has in the meantime 

 increased by growth ; and that the former is removed from the 

 egg in the form of polar bodies a removal which has been ren- 

 dered possible by the occurrence of nuclear division. Hence the 

 formation of polar bodies signified, in my opinion, the removal of 

 the ovogenetic part of the nucleus from the mature egg-cell. Such 

 removal was absolutely necessary, if it is impossible that the ovo- 

 genetic nucleoplasm can be re-transformed into germ-plasm. Hence 

 the former substance cannot be made use of after the maturation 

 of the egg, and it must even be opposed to the commencement of 

 embryonic development, for it is impossible that the egg can be 

 controlled by two forces of different kinds in the same manner as 

 it would have been by one of them alone. I therefore concluded 

 that the influence of the ovogenetic idioplasm must be removed 

 before embryonic development can take place. In this way it 

 seemed to me that not only the ordinary cases of ovogenetic and em- 

 bryonic development became more easily intelligible, but also the 

 rarer cases in which one and the same species produces two kinds 

 of eggs ' summer and winter eggs.' Such eggs not only differ 

 in size but also in the structure of yolk and membranes, although 

 identical animals are developed from each of them. This result pre- 



