249 
on planarice. 
it was reasonable to conclude, that if separating parts be- 
came complete animals, if a mutilated trunk regained the 
defective portion ; and if a head, the most important of all 
organs, was evolved from every inconsiderable fragment, 
supernumerary parts might, by some particular operation, be 
produced ; yet it was long before reiterated trials were re- 
warded with success, and I had almost determined to abandon 
the enquiry, conceiving that a certain nicety, of which 1 was 
not master, should be practised, and that it had been beyond 
my ability to detect the secret cause of failure/' 
Notwithstanding the unpromising commencement of this 
Gentleman's labours he still persevered, and at length noticed, 
that one of the planarize upon which he had made an incision 
a little below the head, had, to quote his own words “ an 
unnatural prominence, which interrupted the general contour 
of the side. October 25th, nearly four weeks after the ope- 
ration, the superfluous reproduction was clearly recognised 
to be the rudiments of a new head. On the 18th of Novem- 
ber the operation of nature was fully accomplished ; a new 
and perfect body crowned by a head had grown out of the 
side of the parent animal, distant about two thirds of the 
total length from the extremity of the tail." 
The work, from which the above extracts have been 
copied, . being now out of print, I have taken the liberty 
of transferring the delineation of this double-headed planaria, 
under a magnified form, to the drawing accompanying this 
paper. Vide Plate XVI. fig. 3. 
With the view of ascertaining whether, in my hands, these 
experiments would prove equally successful, I took the ear- 
