CHAPTER V. 

 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 as a 

 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. 5 1 (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. 5 1 (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 



