OF THE FOETUS, ^91 



heat that is neceflary for the expanfion of 

 the chicken, the embryo, though completely 

 formed, will remain without any motion^ 

 and the organic particles of which it is compo- 

 fed will continue fixed, without beinp- able to 

 give life and motion to the embryo which was 

 formed by their union. Thus, after the motion 

 of the organic particles is flopped, and afcer they 

 have united in fuch a manner as to forrii a foe- 

 tus, fome external caufe is ftill neceffary to give 

 them life and motion. This caufe, or agentj is 

 heat, which by rarifying the fluids, obliges them 

 to circulate : This circulation makes all the or- 

 gans a6t ; and nothing farther is neceliliry for 

 the growth and expanfion of the parts than the 

 continuation of this heat. 



Before the adion of this external heatj there 

 is not the fm.allefl: appearance of blood ; and i 

 never could perceive any change of colour in 

 the veflels till about 24 hours after incubation. 

 In the veflels of the placenta, which cominuni- 

 cate with the body of the foetus, the blood firft 

 appears. But it would feem that this blood lo- 

 fes its red colour, as it approaches the body of 

 tlie animal ; for the chicken is entirely white; 

 and, during the firft, fecond, and third days af- 

 ter incubation, we can with difficulty perceive 

 a few red particles near the animal's body, but 

 which feem not to make any part of it, though 

 thefe red particles are deflined fur the formation 

 of the heart. Thus the formation of blood i:^ 

 an cflect produced by the motion communicated 



T 2 to 



