CHAPTER XVIII 
EXPERIMENTS IN GRAFTING 
ALTHOUGH the process of grafting has long been carried out 
by horticulturists for practical purposes, it is only within recent 
years that the process has been extensively used with animals 
in order to study certain scientific problems. 
Trembley’s experiments in grafting hydra, carried out in 1744, 
are the first, so far as I know, recorded for animals. Later 
Hunter and Duhamel grafted the spur of a cock on the comb, 
where it continued to grow. The experiment is interesting only 
on account of the bizarre nature of the combination. 
The method of grafting carried out with plants is different 
from that practiced with animals in one essential respect; 
namely, that while in plants small buds are inserted in the stock, 
in animals the cut ends of fully formed structures are united, 
and this may sometimes involve the union of cross-sections of 
the entire animal. 
A number of important questions are involved in the results 
of grafting. The main topics to be considered here fall under 
four headings: (x) the facility with which different regions 
can be united; (2) the influence of united parts on each other; 
(3) the possibility of hybridizing by grafting; (4) special prob- 
lems of embryonic development as studied by means of grafting. 
The Union of Different Regions 
The simplest kind of union is that where a part of an animal 
is cut off and put back in place, as when a limb is cut off and 
grafted again in the same position, or as when a worm is cut 
in two and the two parts (complementary parts) are reunited. 
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