336 Metschnikoff on Germ-Layers. [April 
ment has not been altered by the introduction of any obscuring 
disturbance. 
The embryological principles of Darwin were developed in a 
deductive manner by Fritz Miller in his important little book, 
“Fur Darwin,” and were illustrated by many facts chosen from 
the life histories of the lower animals. The way Miiller looked 
at his facts revealed the manner in which the problems of com- 
parative embryology must be approached in future. He es- 
pecially emphasized his belief that individual development only 
repeats genealogical development in cases where the descendants 
(in the course of their embryology) travel without swerving the 
straight path which leads. up to their ancestral form, “ where, 
however, they do not stop, but press farther on.” “In the short 
space of a few weeks or months,” says Miiller, “the ever-alter- 
ing forms of the embryos and larve present to our eyes a more 
or less perfect picture of the changes through which, in the 
course of countless ages, the species has struggled up to its 
present condition.” In connection with Darwin’s ideas on the 
disturbance of the developmental process, Miiller formulated this 
proposition : “ The historical record preserved in the development 
of the individual is gradually lost, since there is always at work 
a tendency to make the path from the egg to the adult as 
straight as possible. The record is, moreover, falsified because 
of the struggle for existence in which the larve that lead an 
independent life have to take part.” 
This book of Miiller’s marked an epoch; and in part under 
its influence there was soon begun a very active overhauling of 
the facts of animal embryology, in which more attention was 
paid to the lower animals than to the higher Vertebrates, Inde- 
pendently of this movement, Kolliker, in 1865 (in the second 
_ part of his “ Icones Histologic,” p. 90), came to certain general 
conclusions, which essentially coincided with the views of Hux- 
dey. “Whatever the cause may be,” says Kölliker, “the uni- 
formity in structure of a Hydrozoan and a young Vertebrate 
embryo is a very striking fact; and if this question is pursued 
further with an eye to the structure and histological development 
of many animals, it is pretty certain that some simple law of de- 
velopment will be discovered. The problem was before long a 
subject of busy investigation. The forsaken theory of the germi- 
nal layers was again taken up in the realm of the Invertebrates, 
