316 Professor Huxley on the Development of Animals. [Feb. 8, 



the structure of the fowl's egg was described, and the effects of incuba- 

 tion were traced, so far .as was necessary to prove that the chick takes 

 its origin from the cicatricula, or blastoderm. 



It was next pointed out, that we owe the discovery of this important 

 fact to the great Harvey, who, in his " Exercitationes de Generatione 

 Aniraalium," demonstrated with perfect clearness, firstly, that the 

 chick is developed from the cicatricula, and not, as had been supposed, 

 from the chalazse, or other parts ; and secondly, that the process of 

 development is an *' epigenesis," or gradual addition of new parts to 

 those already formed. 



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 cen- 

 tury 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 disappear- 

 ance 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 

 Annulosa^ nor in certain Entozoa, 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- 



