EARLIEST STAGES OF THE DEVELOPMENT OF ANIMALS 40! 
In virtue of these discoveries, Harvey has as much right to be 
regarded as the originator of modern embryology, as, in virtue of his 
discovery of the circulation, he has to be considered the founder of 
scientific physiology: but his embryological views met with a less 
fortunate reception than his physiological doctrines ; and for a century 
and a half, the strange dogmas of the evolutionists, supported by the 
vast authority of Haller and of Cuvier, were allowed almost com- 
pletely to override and weigh down the sounder teachings of the 
great Englishman. 
With the publication of Caspar F. Wolff’s “ Theoria Generationis,” 
in the middle of the last century, however, a new epoch commenced ; 
and partly by the labours of that eminent observer, and still more 
largely by those of Pander, Von Bar, Rathke, and Reichert, Harvey’s 
doctrine has been rehabilitated, and has taken its place among the 
firmly ascertained verities of science. 
For want of proper microscopes and other appliances, neither 
Harvey nor C. F, Wolff could trace the origin of the germ further 
back than the blastoderm ; still less could they obtain any just con- 
ception of the essential structure of the ovum. But in the course of 
the last thirty-five years, thanks to the labours of Purkinje, Von Bar, 
Wagner, Bischoff, Wharton Jones, Prevost, Dumas, Coste, and others, 
vast advances have been made. 
It has been ascertained that the ovum of every animal primarily 
consists of a germinal vesicle, containing its so-called spot, and 
enclosed within a yelk, or vitellus; and that, in the great majority of 
cases, the first changes which follow upon impregnation consist in the 
disappearance of the germinal vesicle as such, and the regular division 
of the yelk into smaller and smaller masses, out of which, in one way 
or another, the blastoderm, of which the embryo is a modification, 
arises. Such yelk division, however, has not yet been observed 
among the higher Avnu/osa, nor in certain Extozoa, nor does it occur 
in Pyrosoma. 
So much being definitely ascertained, there is yet one question 
upon which embryologists are widely divided, viz. What is the 
relation between the germinal vesicle and the cells, or structural 
elements, of which the blastoderm is composed? Three answers have 
been given to this question :— 
1. According to the late Dr. Barry, the blastoderm arises from a 
modification of the germinal vesicle, in a manner particularly de- 
scribed by him. No other observer, however, has been able to 
discover a trace of this process ; and it may be regarded as tolerably 
certain that its describer was mistaken. 
VOL. II DD 
