128 PHILOSOPHICAL TRANSACTIONS. [ANNO 1797". 



day no vesicles should happen to be found, it would not be from minuteness that 

 they would escape observation ; therefore should any one be disposed to search for 

 them, he need not bend his sight, as if looking at microscopical objects. 



Valisneri, on the contrary, searched for these eggs with great industry, accom- 

 panied with an ardent wish to find them ; but though his experiments appear to 

 have been judiciously conducted, he never succeeded. Haller also maintains, from 

 a regular series of experiments made on sheep, whose term of utero-gestation is 5 

 months, that some days elapse between the escape of the substance from the ovaries, 

 and the appearance of a circumscribed body in utero, which can properly be called 

 ovum: and that this does not happen until 17 days from impregnation. In the 

 mean time, nothing but irregular masses of mucus are found. The circumscribed 

 form, at this time acquired, seems to depend on the formation of the foetal mem- 

 branes now bounding the contained mucus-like substance. This apparently homo- 

 geneous mass, on the 1 9th day undergoes a change of character ; an opaque spot 

 is seen within it, which subsequent observations prove to be the first evident marks 

 of the evolution or formation of the foetus. From this dim speck of animal ex- 

 istence we may observe a series of regular advances, from an inorganized mucus- 

 like mass to the most beautiful and complicated machine in nature. But to trace 

 her progressive steps through this important work, forms no part of the design of 

 this dissertation. 



The chief difference between De Graaf and Haller on this subject, consists in 

 their opinions respecting the form of the substance that is passing from the ova- 

 ries, whether it is vesicular at this time or not ; for in the subsequent processes 

 they differ but little. No solution can be given of this question by force of reason- 

 ing ; it is from experiment alone that we can receive conviction, notwithstanding 

 the 1 contrary opinions that prevail. All that can be expected from an individual 

 in such a case,* is to add the result of his own labours to one side or the other, so 

 that in the end the preponderance must depend on the weight of evidence. 



The experiments I have made on this simple question do not allow me to incline 

 to the side of De Graaf; for in the rabbit I have never found any thing in the ute- 

 rus which had a regular circumscribed form earlier than the 6th day; and even then 

 the substance was bounded by a covering so very tender, that it scarcely had firm- 

 ness sufficient to support the figure. Before the 6th day, I have never seen any 

 thing but irregular mucus- like masses in the uterus ; but after this time the sub- 

 stance has firmness sufficient to admit of preservation in spirits, a specimen of 

 which I have in my collection of preparations. This acquisition of figure does not 

 depend so much on a difference of consistence, as on the formation of membranes 

 inclosing this substance. These membranes when in a more advanced state of 

 formation, are known by the names of chorion and amnios. The product of con- 

 ception being arrived at this stage, may with some propriety be called an ovum, as 

 it has acquired a determined figure; but the different constituent parts of it are 

 not apparent at this early period ; on the 10th day, in the rabbit, an opaque spot is 



