78 GEACE P. HAYS 



distal extremity. In analysing the possible causes of such con- 

 ditions one must recognize that, although a given case may not 

 be hereditary, it may be the result of conditions identical with 

 those effective in hereditray cases. The difference would be 

 that in the former case the effective factors must have arisen 

 locally, while in the hereditary case such factors were carried 

 in the germ cells. 



The question of when the unknown factors causing Polydactyly 

 or syndactyly may be effective is important in determining what 

 these factors may be and this can be answered only by a considera- 

 tion of the course of normal development. Since no material 

 on the cat is available, a review of the essential points of the 

 development of the human arm will suffice. The lower extremi- 

 ties develop in a way essentially similar to the upper, and the 

 facts regarding the cat may be considered different only in un- 

 essential details. 



The skeleton of the hand first appears outlined in a bedplate 

 of scleroblastema. In this, differentiation proceeds from base 

 to tip, the humerus, radius and ulna being laid down in cartilage 

 before the carpus and fingers. In the hand the metacarpal 

 cartilage forms, then the carpal, and finally the cartilages of the 

 phalanges; the cartilages of the distal phalanges are the last to 

 appear. The first ossification occurs in the long bones. In the 

 hand the distal phalanges ossify first, the metacarpals and proxi- 

 mal phalanges next, then the medial phalanges and finally the 

 carpal bones (Bardeen '10, pp. 366-398). 



As the various parts of the skeletal anlage become differenti- 

 ated, the hand region assumes the form of a rounded pad. It 

 is probably at this or an earlier stage that later poly- or syndactyly 

 is determined. It is difficult to conceive how, after the cartilages 

 had been laid down, certain elements could be eliminated in the 

 case of syndactyly or how new cartilaginous elements could form 

 in the case of Polydactyly. It is quite conceivable, however, 

 that, if this pad were prevented from rounding out to the normal 

 width, differentiation might progress, with a crowding together 

 of the elements such that some would fuse or, in extreme cases, 

 even drop out. On the other hand if the pad were to widen to a 



