286 Exxperimenwuce “Luvewsg y 
A step further involves the union of different regions, and many 
combinations of this sort have been made. It is a point of some 
interest to find that complementary parts unite with no greater 
facility than unlike parts, and even parts with reversed orienta- 
tion unite as readily as do those having the same orientation. 
A series of important experiments in grafting earthworms have 
been made by Joest. The cut surfaces are held together by 
means of two or three ligatures through the skin. In the course 
of a few days the contact surfaces unite with each other, and 
the threads are sloughed off. Parts of different individuals can 
be united as readily as parts of the same worm. Not only com- 
plementary regions can be united, but cut surfaces of different 
levels, and in this way short or long combinations can be made. 
For instance, if the middle region of a worm is cut out, and the 
end pieces grafted together, a ‘“‘short” worm is produced. If 
the union is perfect, no regeneration takes place where the pieces 
have grown together. It also appears that the tail end of the 
short worm does not continue to grow to make the worm 
longer. A “‘long’’ worm may be made by inserting a middle 
piece between the anterior and posterior halves of another 
worm. This combination is also permanent, if the parts neve 
been perfectly united. 
In the preceding cases a posterior cut surface is united to an 
anterior cut surface, 7.e. the pieces have the same orientation. 
It is possible also to unite two anterior cut surfaces or two poste- 
rior cut surfaces. For example, if two tail ends of two worms 
are sewed together by their anterior cut surfaces, a permanent 
union may be effected without subsequent regeneration, al- 
though the combination has two tails and no head, and must 
slowly starve todeath. It is more difficult to unite two posterior 
cut surfaces, not because of any inherent difficulty in the growing 
together of the parts, but because the pieces tend to crawl away 
from each other, and break the ligatures before union of the 
tissues has been effected. 
The results are somewhat different if the cut surfaces are 
brought together so that the median plane of one piece does not 
