102 EBEN J. CAREY 



section is twelve times greater than that of the primordial 

 acetabulum. This fact, together with the muscular restrictions 

 to longitudinal femoral growth, and with the fact that the femur 

 is becoming a more stable bar by the appositional growth of 

 bone at its center of ossification, must be considered in order 

 to understand the continued elaboration of the hip-joint by the 

 femur acting like an electric trip-hammer. 



With these dynamic points in view, the omnipresent puzzle 

 to the student of anatomy, as to the processes by which one 

 segment of a movable joint possesses the socket and the other 

 the ball, is solved. Joints, according to this evidence, are not 

 hereditary; they are the mechanical resultants of the opposed 

 centers of accelerated growth, segmentally distributed in the 

 apparently continuous, blastemal skeleton. 



3. INTERPRETATION 



It is evident that the ultimate differentiation of muscles, 

 joints, and skeletal components from the uniform mass of meso- 

 dermal cells in the hind limb of 10-mm. embryos is dependent 

 upon one of two factors or a combination of both. These factors 

 are, first, an intrinsic self-differentiation of the cells and an 

 extrinsic mechanical interaction due to differential growth. At 

 the present time, the majority of the students of development 

 consider the genesis of the structures enumerated above as a 

 spontaneously hereditary and self-differentiating process, in- 

 trinsic to each cell involved. If such is the case, the solution of 

 the problem of development of the limb goes by default. But 

 the writer takes a decided stand to the contrary. 



Thoma ('07) considered the first formed bone as the resultant 

 of mechanical factors, but the evidence presented was deductions 

 primarily based on the stress and strain of the mature femur 

 supporting the body weight in the erect position. Thoma was 

 right in his mechanical idea, but his evidence was not convincing. 

 The femur is not formed in anticipation of the stress and strain 

 to which it will be subjected in the future, but is an immediate 

 mechanical resultant of the force of its own interstitial growth 

 and the immediate resistances encountered to this growth. 



