THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 637 



ward [and ventrad], the anterior limbs more obliquely than the 

 posterior. In both of them the future extensor side lies dorsal, 

 the flexor side ventral. Both the radial and tibial margins with the 

 thumb and great toe are directed cephalad, the fifth finger and the 

 fifth toe caudad. 



By this and by the fact that the limbs belong to several trunk- 

 segments are explained certain conditions in the distribution of the 

 nerves of the upper extremity. In the case of the arm "the radial 

 side is supplied with nerves (axillaris, musculo-cutaneus), whose fibres 

 are referable to the fifth, sixth, and seventh cervical nerves. Upon 

 the ulnar side, on the contrary, are found nerves (n. cutaneus medialis, 

 n. medius, and n. ulnaris) whose origin from the lower secondary 

 trunk of the plexus discloses their derivation from the eighth cervical 

 and first dorsal nerves " (SCHWALBE). 



In the further course of development both limbs alter their original 

 position, the anterior to a greater extent than the posterior, in- 

 asmuch as they undergo a torsion around their long axes in opposite 

 directions. In this way the extensor side of the upper arm becomes 

 directed backward [caudad]. that of the thigh forward ; radius and 

 thumb are now directed laterad, tibia and great toe mediad. These 

 alterations in position due to torsion are naturally to be taken into 

 account in determining the homologies of the anterior and posterior 

 extremities, so that radius corresponds to tibia and ulna to fibula. 



In the originally homogeneous cell-mass the fundaments of the 

 skeleton and musculature are gradually differentiated from each 

 other, owing to the fact that the cells acquire a more definite 

 histological character. In this connection the following phenomenon 

 is to be observed : 



The parts of the skeleton of the extremity are not all established 

 at the same time, but follow a definite sequence, in somewhat the 

 same manner as, in the development of the axial skeleton, the process 

 of segmentation begins in front and progresses backward. So in 

 the limbs the proximal skeletal elements (i.e., those which are situated 

 nearer to the trunk) are formed sooner than the distal ones. 



This is the most strikingly apparent in the case of the fingers and 

 toes. Whereas the first phalanx has been differentiated from the 

 surrounding tissue in embryos of the fifth and sixth week, the 

 second and third are not at that time distinguishable ; the ends of 

 the fundaments of fingers and toes still consist of a mass of small 

 cells in process of growth. In this mass the second phalanx is then, 

 differentiated, and at last the third. 



