1 94 Dr. Haighton's experimental Inquiry 
Valisneri on the contrary searched for these eggs with great 
industry, accompanied with an ardent wish to find them ; but 
though his experiments appear to have been judiciously con- 
ducted, he never succeeded. 
Haller also maintains, from a regular series of expe- 
riments made on sheep (whose term of utero-gestation is five 
months), that some days elapse between the escape of the 
substance from the ovaries, and the appearance of a cir- 
cumscribed body in utero, which can properly be called ovum : 
and that this does not happen until seventeen days from im- 
pregnation. In the mean time, nothing but irregular masses 
of mucus are found. The circumscribed form at this time ac- 
quired seems to depend on the formation of the foetal mem- 
branes now bounding the contained mucus-like substance. 
This apparently homogeneous mass, on the nineteenth day un- 
dergoes 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 existence 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 ovaries, 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 ques- 
tion by force of reasoning ; it is from experiment alone that 
we can receive conviction, notwithstanding the two contrary 
