GENERATION. Sect. XXXIX. 9. i, 



il parts of tlic fetus, as the brain and heart, with all the upper 

 parts of the body, and mteftines, would feem to be attended with 

 iliil greater difficulties. 



The mi flake of conceiving the embryon to begin its forma- 

 tion in one point only might more readily be fallen into from 

 our habitually confidering an animal as an individual entity ; 

 which it fecms not to be, till an union of the nerves from every 

 part is formed in the common fenforium, and produces a gener- 

 al feuiibility, which is thus diftinguiihed from irritability, 

 which may refide in parts even when detached from the fyftem, 

 as is fcen in the contractions of the heart of a viper taken out 

 oi the body, or of limbs recently cut off. 



2. Another thing difficult to conceive from thofe theories, 

 which fuppofed the firft rudiment to confift of a fmgle entity, 

 was to anfwer the curious queflion,. whether the brain, or heart 

 ;'nd arteries were fir ft formed ; as the motions of the arterial 

 iyftem previously everted feem to have been necelTary for the 

 •cere t ion of fenforial power in the brain, and converfely thofe; 

 motions of the arterial fyftem feem previouily to require 

 the fenforial power derived from the brain. 



This difficulty vanifhes, when we believe, that many parts of 

 tlip young embryon can be begun at the fame time, as various 

 formative fibrils and formative molecules coalefcc, as they come 

 into contact with each other ; and thus the rudiments of the brain 

 and of the heart may be fabricated at the fame in (taut of time. 



3. If fibrils with formative appetencies, and molecules with 

 fdrmative aptitudes or propenfities exift in the circulation both 

 or males and females, why do they not coalefcc there ? This 



ins an unanfvvevable objection to M. Buffon's theory, who 

 holds, that organic particles exift in the circulation ; but in the 



tern above delivered, no organic particles exift in the blood in 

 their combined ftate ; and hence no microfcopic animalcula are 



n in blood recently drawn, though they may appear after fomc 



urs ftagnation ; but the formative fibrils only and formative 



olecules are believed to exift in the circulation j and that they 

 not produce combinations there, as they cannot reft ; and as 



.h combinations would be too large to pafs the capillary vef- 

 fels of the aorta, and of the pulmonary artery, and of all the 



ands, and mult there be perpetually ditlevered, if they could 

 evioufly formed in the larger veiTels. 



4. If limiiar organized particles were fecreted by the fexual 

 mds of the male and alio of the female, why do they not pro- 



Jr.cc parts, or rudiments, of an embryon in the male or female 

 reiervoirs without a reciprocal commixture. This is anotrn 

 lanfwerabk objection to M. Buffon's theory, but not to that 



above 



