PUBOFEMORAL REGION. 



139 



limb bud has appeared, which it begins to do about the end of 

 the 3rd week of development, a section through it reveals a 

 uniform composition of more or less rounded mesoblastic cells with 

 a covering of epiblast (Fig. Ill A). Very soon the central 

 cells near the axis of the bud are densely grouped and form the 

 basis of the humerus. Others arrange themselves to form the 

 biceps, triceps and muscles of the arm ; others form the walls of 

 vessels and the sheaths of nerves. 



mesoblast 



humerus 



deep fascia 



B 



Fig. 1W.—A. Diagrammatic section of the Arm Bud of an embryo at the com- 

 mencement of the 4th week. 

 B. Corresponding section of the Adult Arm. 



After these various groups of cells have become differentiated, 

 there are numerous cells left over which form a basis in which 

 the specialized cells and groups of cells are packed and en- 

 sheathed. The undifferentiated mesoblast forms the connective 

 tissue or fascial system of the part. From the manner of its 

 origin it is evident that the connective tissue system — the fasciae 

 and septa — must form a continuous formation of sheaths, each 

 being in continuity with that of every surrounding structure. 

 The sheaths of the biceps, triceps and brachialis anticus, the 

 periosteum of the humerus, the deep fascia, internal and external 

 intermuscular septa, the sheaths of the vessels and nerves of 

 the arm, represent the mesoblastic tissue which was left over 

 after the structures which they enclose were differentiated, and 

 are, from the manner of their origin, necessarily in continuity. 



