AQUATIC MAMMALS 



cartilage, and the latter is undoubtedly never differentiated at all in 

 these mammals, so that there is no process of changing over of a synovial 

 joint into a syndesmotic or fibrous one, but rather a later, direct altera- 

 tion of the interphalangeal perichondrium or periosteum into fibrous 

 tissue as there develops need for interphalangeal strengthening. In 

 effect, then, the phalangeal articulations of the Cetacea are in the na- 

 ture of synarthroidal (rather than synchondrosial) joints or sutures, 

 differing from most sutures as we know them only in that they occur 

 between cartilages rather than bones, at least until old age. 



The subject of hyperphalangy in the cetacea has been productive of 

 a perfect furore of argument and the most violent partisanship for one 

 theory or another. The theses that have been advanced in the endeavor 

 to explain this condition may be said to number five, and to these may 

 be added a sixth, which hardly seems promising, and a seventh consisting 

 of the theory to which I incline. 



(1) Leboucq (1889) considered that the Cetacea are descended 

 directly from swamp-inhabiting creatures on the order of amphibians, 

 rather than from a terrestrial mammalian stock ; hence that hyperphalangy 

 has been directly inherited from this ancestor. Presumably, then, he 

 would consider that this condition can be explained in the same way 

 that Howes has argued (in 3) . 



(2) Steinmann (1912) advanced the belief that the Cetacea are 

 derived directly from the ichthyosaurs rather than from land mammals. 

 It was therefore necessary that he consider that hyperphalangy has been 

 inherited from these reptiles, which does not explain its origin. 



(3) Howes and Davies (1888) considered that hyperphalangy has 

 been attained by means of the independent ossification of intercalary 

 syndesmoses. 



(4) Kiikenthal (1889) was persuaded that this condition was 

 brought about through the initial separation and subsequent independent 

 development of the phalangeal epiphyses, and a number of others have 

 favored this view. 



(5) Weber, Bauer and others have held that hyperphalangy was 

 initiated through the secondary division of a predigital strand of carti- 

 lage, presumably on the order of those present in the Otariidae. 



(6) There may be added to this group the rather unlikely theory 

 that supernumerary digital elements have occurred through the funda- 

 mental division of the phalangeal anlage. 



(7) A further premise, and one which to me seems to hold the 

 r^rKt promise, is that hyperphalangy was initiated by the addition of 



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