IV.] FOUNDATION OF A THEORY OF HEREDITY. 221 



the cell-body ; but the so-called centres of attraction at the 

 poles of the nuclear spindle obviously arise under the influence 

 of the nucleus itself, even if we admit that they are entirely 

 made up of cytoplasm. But this point has not been decided 

 upon, and w^e may presume that the so-called ' Polkorperchen ' 

 of the spindle (Fol) are derived from the nucleus, although they 

 are placed outside the nuclear membrane '. Many points con- 

 nected with this subject are still in a state of uncertainty, and 

 we must abstain from general conclusions until it has been 

 possible to demonstrate clearly the precise nature of certain 

 phenomena attending indirect nuclear division, which still 

 remain obscure in spite of the efforts of so many excellent 

 observers. We cannot even form a decided opinion as to 

 whether the chromatin or the achromatin of the nuclear thread 

 is the real idioplasm. But although these points are not j^et 

 thoroughly understood, we are justified in maintaining that the 

 cell enters upon division under the influence of certain con- 

 ditions of the nucleus, although the latter are invisible until 

 cell-division has already commenced. 



I now pass on to examine my hypothesis as to the significance 

 of the formation of polar bodies, in the light of those ascertained 

 facts which bear upon it. 



If the expulsion of the polar bodies means the removal of the 

 ovogenetic nucleoplasm after the histological differentiation of 

 the egg-cell is complete, we must expect to find polar bodies in 

 all species except those in which the egg-cell has remained in 

 a primitive undifferentiated condition, if indeed such species 

 exist. Wherever the egg-cell assumes the character of a 

 specialized cell, e.g. in the attainment of a particular size 

 or constitution, in the admixture of food-yolk, or the forma- 

 tion of membranes, it must also contain ovogenetic nucleo- 

 plasm, which must ultimately be removed if the germ-plasm 

 is to gain control over the egg-cell. It does not signify at 

 all, in this respect, whether the ^gg is or is not destined for 

 fertilization. 



If we examine the Metazoa in regard to this question, we 



1 E. van Beneden and Boveri have recently, quite independently of each 

 other, made a more exact study of these ' PolkOrperchen ' (; Centrosoma.' 

 Boveri;. They show that nuclear division starts from these bodies, al- 

 though the mode of origin of the latter is not yet quite clear.— A. W., 1888. 



