4 i6 HEREDITY AND DEVELOPMENT 



and are to be seen being foimed. There is no " evolutio " ; 

 there is new formation or " epigenesis." But how a germ that 

 seems to start anew every time " from the pit of material homo- 

 geneity " can develop as it does, the upholders of epigenesis 

 could not tell. 



In fact, the preformationists and the believers in epigenesis 

 came to a dead-lock, and both schools usually fell back on the 

 assumption of hyperphysical agencies. Until the genetic or 

 germinal continuity which links generation to generation was 

 realised, there could be no real progress in theories of develop- 

 ment. 



The New Evolutio and Epigenesis. — With the growth of 

 embryology the whole venue has changed, and it would be 

 misreading history to say that students of development are 

 still facing the dilemma expressed in the opposition between the 

 eighteenth-century schools of evolutio and epigenesis. Yet there 

 is to-day an analogous antagonism, which we must briefly discuss. 



The Mosaic Evolutio Theory. — On one view it is supposed 

 that the germ-cell has an architectural organisation, prede- 

 termined before development begins, and that development is 

 in part a " histogenetic sundering " of the pre-existing germinal 

 mosaic. Some authorities have suggested that the predeter- 

 mination is in the organisation of the egg- cytoplasm— the 

 central idea of the theory of " organogenetic germinal areas " 

 which His elaborated in 1874. This theory may find support 

 in experiments such as those of Roux on the frog ovum, in 

 which one of the first two cleavage-cells was punctured, and 

 its intact neighbour developed into a one-sided embryo ; though 

 the edge is taken off this case by the observation of Hertwig 

 that in other conditions the intact blastomere may develop into 

 a complete embryo of half the normal size. T. H. Morgan has 

 shown that if the ova experimented with are kept stationary 

 the result observed by Roux is likely to be seen, while if they 

 are allowed free movement in the water the result observed 





