1892.] NATURAL SCIENCES OF PHILADELPHIA. 223 



guments at i, i, i, toward the vertebral bodies V, V, Y. In this 

 manner will be developed the imbrication indicated by the heavy 

 border along the posterior margins of the scales s, s, s, in Fig. 1, 

 and in Fig. 3, in longitudinal section through the scale sacks or 

 pockets at s, s, s. 



It will be clear that in the case considered the arrangement and 

 imbrication of the scales is determined by the actions of the 

 segmentally arranged muscles of the body. In other words, what- 

 ever has determined the development of somites has also, in the 

 most clear and direct manner, detei'mined the segmentally recurrent 

 and peculiar tri-linear and imbricated arrangement of the scales of 

 many fishes. It may be urged in objection that heredity has 

 determined the number, arrangement and the development of the 

 somites and, therefore, the development of the scales is also a sequence 

 of hereditary influences working thus indirectly. This view of the 

 case may be admitted without invalidating the conclusion that, given 

 the growing mechanism here described, the development of the scales 

 would under any circumstances have been interfered with at the 

 points where the integument was being continually flexed, wrinkled 

 or folded as it is around the integumentary areolae wherein the 

 scales are formed, as has been here proved to correspond with the 

 facts. 



Special types of squamation amongst fishes may require an inter- 

 pretation different as to details from the foregoing, but it is probable 

 that such special cases will rather tend to confirm than otherwise 

 the views developed in this sketch of an hypothesis respecting the 

 mechanical origin of the arrangement and imbrication of the scales 

 of fishes. For example, one of the most extreme cases, that of the 

 sturgeon, shows that the smaller integumentary plates between the 

 large dorsal, lateral and ventral rows, conform to these lines of 

 tension of the myocommata upon the integument. An even more 

 instructive example is that of the common eel in which the scales 

 are oblong rhombs or parallelograms, arranged with their diameters 

 in oblique lines, running in two directions conformably with the 

 tensions, wrinklings and foldings of the integuments j^roduced by 

 the oblique insertions of the muscles when the latter are brought 

 into action. Other cases where the scales are very fine might be 

 urged in objection, especially where several oblique rows of scales 

 are found to correspond to each somite. Such parallel duplication of 

 scale rows, however, does not invalidate the principle since the rows 



