306 LECTURES ON BIOLOGY 



But how is it that the organism can produce germ-cells, i.e., 

 cells which contain the complete germ-plasm with all the deter- 

 minants? Just as a cell cannot originate otherwise than by 

 fission of an already existing cell, so specific determinants are 

 unable to create themselves anew. Tf we regard abiogenesis 

 of the simplest ' biophores ' conceivable we may with the same 

 right believe that the lowest organisms owed their existence to 

 abiogenesis. It follows that the reproductive cells can only be 

 formed where as yet all determinants of the relevant kind 

 are present, co-ordinated in ids. That is to say : in addition 

 to heterokinesis of the egg-cell there must also take place 

 homoeokinesis. This also is said to be actually the case, and 

 certain cells resulting from a division of the ovum are said to 

 receive a small part of the unaltered germ-plasm which they 

 transmit in a similar manner to their descendants. These cells 

 then become the stem-mothers of the sex-cells. In other words, 

 there are certain ' germ-paths ' along which the direct trans- 

 mission of the total hereditary substance proceeds from genera- 

 tion to generation. Thus, in spite of heterokinesis, the continuity 

 of the germ-plasm remains preserved ; the descendants possess 

 in their reproductive cells the same number of determinants 

 which accumulated from the parents in the egg-cell during 

 fertilization, and transmit it afterwards to subsequent generations. 

 Tn Weismann's opinion we must therefore distinguish between 

 the germ-plasm on the one side, and the body or somatic cells 

 on the other. 



It cannot be denied that on the basis of Weismann's germ- 

 plasm theory the marvellous processes of differentiation during 

 the evolution of the egg-cell to the complete organism fini a 

 simple and satisfactory explanation. By distributing the deter- 

 minants of the hereditary substance by heterokinesis among 

 the different descendants of the ovum, and thus determining the 

 characters which the individual cells are to assume, the process 

 of mitotic nucleus-division would represent ihe means of determin- 

 ing the form. But though a solution would be most acceptable, 

 this hypothesis is confronted with serious objections : for without 

 taking refuge in supernatural, uncontrollable forces, it is incon- 

 ceivable how the same process of indirect nucleus-division can 

 now lead to an absolutely just and equal division of the heredi- 



