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before mentioned) that if this blood had been an uniformly clear 

 liquor, its motion could not by any means have been difcovered ; but 

 as itconfuled of a very tranfparent fluid, mixed, as it I'eemed, with 

 globules of different iizes, which were plainly to be diftinguilhed, 

 though they had not, as yet, in this creature acquired any colour, 

 the circulation was thereby rendered very apparent. 



When thefe tadpoles were about eight or ten days old, I could 

 perceive a fmall particle moving within their bodies, which I conclu- 

 ded to be the heart ; and the fluid which was protruded from it 

 began to afl'ume a red colour. 



Upon examining the tail of this creature, a hght prefented itfelf,. 

 more delightful than any that my eyes had ever beheld ; for here I 

 difcovered more than fifty circulations of the blood, in different 

 places, \\'hile the animal lay quiet in the water, and I could bring it 

 before the microfcope to my wifli. For I faw, not only that the 

 blood in many places was conveyed througli exceedingly minute 

 velTels, from the middle of the tail towards the edges, but that each 

 of thefe veflels had a curve, or turning, and carried tlie blood back 

 towards the middle of the tail, in order to be again conveyed to the 

 heart. Hereby it plainly appeared to me, that the blood-vedels I 

 now faw in this animal, and which bear the names of arteries and 

 veins, are, m fadt, one and the fame, that is to fay, that they are pro- 

 perly termed arteries fo long as they convey the blood to the fartheft 

 extremities of its veflels, and veins when they bring it back towards 

 the heart. For example, I fee many blood-vefl'els in the tail of a 

 tadpole taking their courfe, as reprefented in Plate IV. fig. i. ABC, 

 where the pofition of the parts A and C is towards the fpine or mid- 

 dle of the tail, and the part B towards the edge of it. In A B, the 

 blood is driven from the heart, and in B C, it is brought back again, 

 and thus may we fay, that the veflel ABC, is both an artery and a 

 vein, for it cannot be denominated an artery, farther than where the 



