66 C. J. HERRICK AND G. E. COGHILL 
We have a tolerably complete picture of the steps in the 
physiological development of the swimming reflex in response to 
external stimulation in the case of Diemyctylus and Amblystoma 
(Coghill, 09) and more recent anatomical work has revealed to 
us some of the details of the inter-neuronic connections involved 
in these reactions at successive stages of complexity. These 
ontogenetic studies are important in themselves in that they 
give an insight into the mechanism of functional differentiation, 
and it may be that they will also contribute something to our 
knowledge of the factors involved in phylogenetic differentiation. 
Fig. 1 Diagram of a simple reflex arc illustrated by a dorsal and a ventral 
root neurone of the spinal cord. After Van Gehuchten (slightly modified). 
The Amphibia present peculiarly favorable materials for stud- 
ies of this character, for the embryo becomes functional at a very 
early stage of differentiation, in this respect contrasting sharply 
with higher vertebrates, where organ formation and histogenesis 
are much further advanced before functional activity is possible. 
In amphibian larvae we are, therefore, able to watch the process 
of tissue differentiation during all of its later stages parallel 
with the correlated changes in behavior, and thus to avoid many 
pitfalls in the interpretation of the data. The steps in the 
ontogenetic development of the mechanisms of some typical 
reflexes of Amblystoma tigrinum will now be described. 
