102 'Journal of Comparative Neurology and Psychology. 



of tlie arrangement of the special cntancous nerves of fishes and 

 amphibians is obvious. It is not to be supposed that the cephali- 

 zation of the locomotor effectors is, in any respect, a direct cause 

 of the cephalo-caudal migration of the special cutaneous receptors 

 and conductors, but such a cejihalization would certainly favor 

 the development of such systems, for, as already suggested, their 

 peripheral conductors hold essentially the same relation to the cephalic 

 part of the central system as do the most primitive central conductors 

 from the trunk. 



It should be noted here that a certain amount of locomotion may 

 be acquired by an amphibian embryo by other movements than the 

 S reaction as described above. The body may be flexed, for in- 

 stance, and straightened by a series of secondary, vibratory move- 

 ments. Such a reaction propels the animal on its side in a circle 

 or spiral path. Also, a rapid succession of reversed flexures, in 

 which no S reaction can be detected, may give swimming in a zigzag, 

 erratic course. But normal, upright swimming in a direct course is, 

 according to my observations, attained only through the perfecting 

 of the S reaction and its performance in series. 



As already suggested, this development of the swimming movement 

 is of interest from the point of view of animal behavior. We now 

 see that swimming, which may be regarded as instinctive in these 

 forms, arises as the elaboration of the simplest known reflex in the 

 embryo, the contraction of the most cephalic trunk muscles. Certain 

 forms of the flexure, such as the TJ reaction and the coiled reaction, 

 do not seem to be in the direct line of the development of tlie swim- 

 ming movement, being simply intensive or tetanic forms of the or- 

 dinary flexures. On the other hand, the other types of flexure 

 develop in a regiilar order and in a remarkably constant manner 

 into the movements of locomotion. Kow none of these simple flex- 

 ures can be regarded as having any value as trials, since the Diemyc- 

 tylus swims perfectly upon leaving the egg membranes in the normal 

 course of development, and within them it can gain no practical ex- 

 perience for swimming out of movements of any sort. Instinctive 

 swimming, therefore, and the simplest reflex alike, are inherent in 

 the neuro-muscular system of the embryo, and while the former de- 



