CHAPTER-Sy: 
DICHOTOMY, 
A STRONG tendency exists in the animal and vegetable 
kingdoms for parts ending in free extremities to bifurcate 
or dichotomize. In many instances partial or complete 
dichotomy occurs so constantly that it is regarded asa 
normal condition. Many extraordinary and beautiful 
forms among animals depend upon its occurrence, as 
well as a large number of malformations when dichotomy 
occurs abnormally. 
The principle may be illustrated by the star-fish. In 
many specimens of this invertebrate we find normally 
five arms arranged radially around a central disc. It is 
not uncommon to find, as in fig. 51 (A), an arm redupli- 
cated—this is complete dichotomy, and as the two halves 
of the bifurcated ray are symmetrical we speak of it as 
equal dichotomy. On the other hand, the ray may be 
double only at its extremity. In fig. 51 (B) is a star-fish 
ray drawn upon a larger scale in which the dichotomy is 
only partial. 
Every supernumerary ray attached to star-fish is not 
due to dichotomy. When an arm is lost by accident 
and the corresponding segment of the disc is uninjured, 
or only slightly damaged, one or more rudimentary rays 
may grow from it. Such rays are produced by a process 
