Dr. Clarke’s Account of a Tumour 
resemblance is so close, that the latter may be said to be de- 
monstrated by the former. 
A certain temperature, nourishment, and the application of 
vital air, (or oxygen,) seem to be essential to the evolution of 
the young of oviparous animals. 
As the young are expelled from the mother, contained in the 
cavity of the egg, at a very early period of their existence, and 
as afterwards they have no connection whatsoever with her, 
these are supplied by various contrivances ; and the mode of 
application has been very distinctly explained, by modern in- 
quirers into the structure of eggs. 
Since then the same substances are to be produced, and sup- 
ported, in viviparous as in oviparous animals, the conclusion is 
reasonable, that similar means should be employed to attain 
similar ends. 
It is easy to conceive how warmth may be imparted to a 
foetus situated in the uterus. 
The materials for nourishment, it receives from the placenta; 
but the precise manner in which they are supplied has not yet 
been discovered. Of the fact there can be no doubt, because 
there are many cases on record, in which there could be no 
other possible way by which support could be had.* 
With respect to vital air, (or oxygen,) the young of all vivi- 
parous animals, whilst in the uterus, live in the same medium 
as fishes, and have a structure similar to gills, for the exposure 
of their blood to it : this structure is the placenta. 
The heart of the foetus is adapted to this mode of life, and 
in effect consists but of one auricle and one ventricle, as it is 
* A case of this kind I described some time ago, which is published in the Philoscv* 
phical Transactions for the year 1793; 
