AQUATIC MAMMALS 



and some would not. This should invariably be more marked in the 

 embryo, but no indications of it have ever been discovered. 



(3) Weber, Bauer and others have subscribed to the behef that 

 hyperphalangy has been accomphshed by the segmentation of a predigi- 

 tal strand of cartilage. It has been mentioned that in Ornithorhynchus 

 the part of the webbing of the manus that projects beyond the nails is 

 slightly stiffened by linear thickenings of the skin, running in continua- 

 tion of the digits. These may be partly or wholly subcutaneous in 

 situation, however. Of course skin and cartilage is very different in 

 structure, but it is well known that where there is real need for the 

 stiffening of a membraneous extension of the skin in mammals this will 

 be accomplished, where practicable, by rods of bone or cartilages (as 

 in flying squirrels and bats) . Hence it is reasonable to suppose that the 

 predigital cartilages of the sea-lions had their inception in just this 

 manner. And where there is well formed cartilage, ossification may, 

 without difficulty, eventually take place. The developmental process 

 involved in the initiation of predigital cartilages is, of course unknown, 

 but it cannot reasonably be doubted that this was intimately correlated 

 with the fact that there was no longer the stimulus furnished by repeated 

 contact of the tips of the true digits with a hard surface. There were 

 other factors concerned, however, as indicated by the accomplishment 

 of digital extension in bats and marine turtles by means of elongation 

 of the bones. There seems to be nothing particularly remarkable in this 

 process. Rather would the incomprehensibility lie in the question of 

 how the cartilaginous strands could become segmented so as to develop 

 separate centers of ossification for the formation of phalangeal elements 

 precisely like those of the true digits. One would reasonably expect 

 that the ossicles situated distad of the latter would exhibit some abrupt 

 transition in character, and even if this modification originally took place 

 so far in the past that this transition was not apparent in the adult, it 

 would almost certainly show to a striking degree in some embryonic 

 stage. 



(6) Unlike the case of the axial skeleton the bones of the appendages 

 develop from somatic, mesenchymal condensations. Occasionally, from 

 unknown causes, some of these anlage may experience reduplication. 

 This is known because sometimes .one hears of a man, cat or pig with 

 six or more well-formed fingers, or one phalanx more or less than 

 normal upon each digit. This, apparently, is different from the process 

 known as twinning, in which the tip of the finger is split, and even the 

 second phalanx may be double. A sixth finger should not be looked 

 [ 264 } 



