concerning Animal Impregnation. 161 
elaborate and evolve the different organs of which the new 
animal is composed. 
To these facts we may add, that the calyx or capsula which 
formed the parietes of the vesicles, thickens, by which the ca- 
vity is diminished. This cavity, together with the opening 
through which the foetal rudiments escaped becomes oblite- 
rated, and from the parietes of these vesicles having acquired 
a yellowish hue, they are called corpora Intea. 
But though some important facts are clearly ascertained, 
there are others still problematical. Physiologists are by no 
means agreed concerning the immediate cause of conception. 
All admit the necessity of sexual intercourse. They acknow- 
ledge too the necessity of some part of the female being affected 
by the direct contact of a fecundating fluid, but what the pre- 
cise part is which must receive the stimulus, has hitherto been 
involved in mystery and doubt. Nor are they more unanimous 
respecting the state or condition of the substance that passes 
from the ovaries ; whether at the time of its expulsion it has a 
circumscribed vesicular character, or whether it has no deter- 
mined figure. De Graaf and Malpighi, in the last century, 
and some respectable physiologists of the present day, adopt 
the first opinion ; Haller and some others favour the last. 
The subject of conception involves other problematical points 
not less interesting; the discussion of which I purpose waving 
at present, in order the better to direct my attention more 
closely to the preceding questions. 
The intention then of this essay is to explore the proximate 
cause of the impregnation of animals, and to trace with more 
accuracy the visible effects of it from their, first appearance, 
until the rudiments of the foetus are lodged in the uterus, and 
MDCCXCVII. Y 
