293 Ur Allen Tliomson on the Vascular System. 



has the resemblance between them appeared to be. Since our ac- 

 quaintance with the development of the ovum has increased, the 

 mode in which the organs of the foetus are produced, has been 

 proved to be analogous in the different classes of vertebrated ani- 

 mals ; and we are induced to believe, that a knowledge of the 

 simpler forms of these animals must greatly facilitate our study 

 of the more complicated. 



For some time even after the development of the chick, as 

 well as of the foetus of several other animals, had attracted con- 

 siderable attention, physiologists believed that the impregnated 

 ovum, at the time it passes from the parent, and before the pro- 

 cess of development has commenced, contained already formed, 

 all the parts which are afterwards to be found in the perfect 

 animal. They seemed to make a duty of conceiving that the 

 successive appearance of the organs of the animal which is ob- 

 served during the process of development, is owing simply to the 

 evolution or coming into sight of parts, the transparency and 

 small size of which had before rendered them invisible ; and in 

 no instance did they permit themselves to imagine, that the ac- 

 tual formation of an organ, or the union and arrangement of its 

 parts, could be observed to proceed before their eyes. As their 

 knowledge of development increased, it is true, many observations 

 were made, which were with difficulty to be explained in accord- 

 ance with the commonly received opinion ; but still, the theory 

 of pi'eformation and simple evolution of parts was strenuously 

 defended by some of the most distinguished physiologists, till it 

 was opposed by C. F. WoliF, in a Thesis * which he published 

 at BerUn in 1759. 



This celebrated author endeavoured to show, by a reference 

 to many accui*ate observations which he had made on the de- 

 velopment of the foetus of the bird, that the impregnated egg, 

 before incubation has commenced, is very simple in its structure, 

 that it does not, so far as we are able with the greatest attention 

 to discover, contain any part that can be compared to the foetal 

 or adult animal, and that several of the organs of the chick can 

 be seen gradually forming by the apposition of their parts. 

 WolfF carefully avoided entering into any speculation on the 

 causes of the phenomena which he observed, and affirmed that it 



• Theoria Generationis. Republished 1774. 



