334 Metschnikoff on Germ-Layers. [April 
METSCHNIKOFF ON GERM-LAYERS:.: 
TRANSLATED BY H. V. WILSON. 
Al of the most fundamental principles of the comparative 
embryology of to-day can be traced back, with a greater or 
less change of form, to the beginning of the century. Among 
such is the idea embodied in the following law, which was enun- 
ciated by the school of natural philosophers in Germany: “The 
evolution undergone by every animal from the beginning of its 
life corresponds to the evolution which is to be observed in the 
Series of animals.” The law as thus stated met with opposition 
from Von Baer (1),? who maintained that the embryonic stages of 
an animal are by no means to be compared with other adult forms, 
but with the embryos of these forms. He had finally to admit, 
however, that the difference between these two views is not 
nearly so great as it appears to be at first sight. 
lready animated by this philosophical generalization, embry- 
ology received a great stimulus from the parallel Louis Agassiz 
drew between the embryonic stages of existing animals and the 
main stages in the geological development of the animal king- 
dom. Agassiz himself failed to make the right deductions from 
this truth. 
While students of embryology were thus engaged in looking 
for general points of similarity, on the one hand, between embry- 
onic stages of animals and existing adult forms, and between 
embryos and extinct animals on the other, Huxley, in 1849, insti- 
tuted the comparison between the germinal layers of Vertebrates 
and the layers of the Ccelenterate type. To the latter he gave 
the names of ectoderm and endoderm. This idea did not re- 
main unnoticed in England, but was generalized and given a 
popular character by Herbert Spencer in one of his beautiful 
essays, entitled “ The Social Organism.” Let me quote from the 
English philosopher: “ Throughout the whole animal kingdom, 
from the Cælenterata upward, the first stage of evolution is the 
same. Equally in the germ of a polype and in the human ovum, 
the aggregated mass of cells out of which. the creature is to 
* The following paper forms the closing chapter of Professor Elias Metschnikoff’s 
 “ Embryologische Studien an Medusen,” Wien, 1886. 
2 Th ab os this article c ah bibliographi 1 list appended. 
