VI. 



Hertwig's "Preformation or New Formation."' 



Part III. 



DR. HERTWIG entitles the concluding part of his paper, 

 "Thoughts Tending to a Theory of Development of Organisms." 

 Ke professes, not to work out a theory in detail, but to indicate the 

 direction in which guidance is to be obtained. Let it be remembered 

 that in his criticism of Weismann's preformationist views there were 

 two main points. First, he thinks not only that Weismann has failed 

 to prove the existence of " heirs-unequal division," but that there are 

 many grounds for supposing that all cell-division is a case of " heirs- 

 equal " division. Hence there is no reason to suppose that the 

 various elements of the germ-plasm are marshalled to their places in 

 the course of an individual development, and that this sorting out of 

 preformed determinants is the reason why an egg produces with im- 

 perative fidelity the thousand different cells of the adult. Every cell 

 of the whole organism, so far as inherited dower goes, is identical. 

 Next, Hertwig throws doubt upon the whole conception of determi- 

 nants, pointing out that very many of the individual characters of 

 organs and of organisms are not properties of cells at all, but are 

 properties of aggregates of cells, and therefore cannot come into 

 existence until aggregates arise, and certainly can have no preformed 

 determinant within a single cell. 



Hertwig is disposed to steer a middle course between preforma- 

 tion and epigenesis, to cull from either theory what seems reasonable 

 and in consonance with observed fact, and to reject much of the 

 speculation with which both preformationists and epigenesists have 

 rounded off their conception of Nature. As yet, we are far from 

 having a complete explanation of the problems, but it seems to him 

 as if Nature were a follower neither of Weismann nor of his oppo- 

 nents, but evenly pursued a middle way. 



It is necessary to suppose that each organism starts with a 

 specific dower. However much can be attributed at present, or may 

 come to be attributed, to the moulding forces of environment, there 



1 Zeit- und Streitfragen der Biologie. By Professor Dr. Oscar Hertwig. 

 Pamphlet I. Praeformation oder Epigenese ? Grundziige einer Entwicklungs- 

 theorie der Organismen. Pp. 144, with 4 illustrations in the text. Jena : Gustav 

 Fischer, 1894. Price 3 marks. 



