FOUNDATION OF A THEORY OF HEREDITY. 217 



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 yet thoroughly 

 understood, we are justified in maintaining that the cell enters 

 upon division under the influence of certain conditions 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 formation of membranes, it must also 

 contain ovogenetic nucleoplasm, 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 egg is or is not destined 

 for fertilization. 



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

 that polar bodies have not yet been discovered in sponges x , but this 

 negative evidence is no proof that they are really absent. In all 

 probability, no one has ever seriously endeavoured to find them, and 

 there are perhaps difficulties in the way of the proofs of their exist- 

 ence, because the egg-cell lies free for a long time and even moves 

 actively in the tissue of the mesogloea. We might expect that the 

 formation of polar bodies takes place here, as in all other instances, 

 when the egg becomes mature, that is, at a time when the eggs 

 are already closely enveloped in the sponge tissue. At all events 

 the eggs of sponges, as far as they are known, attain a specific 



1 The existence of polar bodies in sponges has been recently proved by Fiedler : 

 ZooL, Anzeiger., Nov. 28, 1887. A. W., 1888. 



