Hertwig 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.” 18° 
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, 
18Qscar Hertwig: Die Zelle und die Gewebe. II. P. 273, 274, 
276. 
