Her twig 241 



only by division. It follows that in all living beings onto- 

 geny must commence by the cleavage process." 



"An organism constituted by layers and groups of 

 cells disposed in a definite order can be formed from a 

 heap of cells only on condition that the cells while they 

 are multiplying, begin to be arranged in separate assem- 

 blages and so progress in accordance with certain rules 

 from the very simple initial forms to more complex ones. 

 Thus the gastrula implies as prerequisite the simpler 

 stage of the germinal vesicle. Thus the embryonic cells 

 must first be disposed in germinal layers, which constitute 

 the basis for further processes of differentiation in their 

 territory. The anlage of an eye in a vertebrate can be 

 formed only after a nerve tube has been separated from 

 the outer germinal layer, since in it is included also the 

 material for the formation of the optic vesicles." 



"Certain forms then become firmly fixed in the 

 developmental process, despite all the constantly acting, 

 modifying factors, because it is only by means of them 

 that the complicated final state can be reached in the most 

 simple way and in the most suitable manner." 185 



Thus as we said, Hertwig seems in this really to sup- 

 pose, contrary to what he asserted above, that the idio- 

 plasm is not at all a heaping up of numerous anlagen 

 representing respectively the successive steps of the evolu- 

 tion of the species, but so transforms itself with each new 

 phyletic acquisition that it preserves no trace of preceding 

 phyletic states. 



In this he is in complete accord with the hypothesis 

 of Spencer, from which in fact he quotes long passages 

 and makes them his own. And accordingly he supposes, 



185 Oscar Hertwig: Die Zelle und die Gewebe. II. P. 273, 274, 

 276. 



