96 A Sketch of Comparative Emébyrology. (February, 
A SKETCH OF COMPARATIVE EMBRYOLOGY, 
I—THE HISTORY OF THE GENOBLASTS, AND THE THEORY OF SEX. 
BY CHARLES SEDGWICK MINOT. 
HE series of articles, of which this is the first, is intended to pre- 
sent in a simple and popular manner, the leading results of the 
very numerous researches upon the development of animals, pub- 
lished during the last fifteen years. These researches have com- 
pletely altered the whole science of comparative anatomy and 
animal morphology, by entirely upsetting a large part of Cuvier’s 
classification and the idea of types upon which it was based, sub- 
stituting the demonstration of the fundamental identity of plan 
and structure throughout the animal kingdom from the sponges 
to man. The details of the observations are already too many 
for any but the most industrious specialist to become familiar with. 
We have now entered upon the period of generalizations, which 
are already so numerous and important that it is impossible to 
study scientific zodlogy without some knowledge of them. This 
great progress is still so recent that its results have not been 
transferred to the text-books, nor even gathered together in any 
scientific review. Nevertheless, it is possible to compile an out- 
line which may be accepted as fairly correct. This outline it is 
my present object to trace, with the hope that it may prove at 
once accurate and useful. 
Each branch of the subject will be treated by itself. The iilus- 
trations will be taken mainly, if not exclusively, from pen draw- 
ings prepared by myself for a work on Comparative Histology, 
upon which I am engaged. The original source of each figure 
will be given. 
The arrangement of topics willbe, first the structure of the egg 
and spermatozoén; second, the phenomena of impregnation ; 
third, segmentation and the formation of the germ-layers; fourth, 
the essential features of the embryology of the leading animal 
types, beginning with the simplest and ascending to the most 
complicated and highest. ; 
The starting point of comparative embryology, as indeed of 
nearly all branches of biology, is the cell, composed of the nucleus 
surrounded by protoplasm. A few years ago the theory was ad- 
vanced that the nucleus was unnecessary, and various observa- 
tions were adduced to show that in a considerable number of 
