416 



(5) The Leg Muscles. 



The essential features of the development of these muscles 

 are similar to those observed in the head muscles. They 

 need, therefore, to be referred to only briefly here. 



Excepting the tarsal muscles, for the present, the leg 

 muscles are in the form generally of two sets in each segment 

 of the leg (fig. 16). Of these one pulls the segment which it 

 moves in one direction, the other in the opposite (fig. 18). 

 Since, moreover, the joints are of such a nature as to limit 

 the extent of movement in one of these directions, while 

 the muscles are so disposed as to cause only motion in one 

 particular direction, for each muscle, it becomes possible to 

 speak of the one as a flexor muscle, and its antagonistic one 

 as the extensor, the extensor being that one whose activities 

 are limited by the peculiar mode of articulation between the 

 segments. 



Of these muscles, a pair, the flexor and extensor tarei, 

 are developed in the tibia, and in the femur the correspond- 

 ing muscles of the tibia are developed. In the trochanter only 

 one muscle, the extensor fomoris, is formed (fig. 17). The 

 coxa contains the flexor femoris, as well as, apparently, cer- 

 tain other muscles (fig. 16). Proximally these muscles are 

 all spread out over a large part of the segment, while their 

 distal portions converge and are attache'd to a tough tendon 

 fibrillated in structure, which is inserted upon the upper part 

 of the next segment. This suggests, of course, a mode of 

 development similar to that observed in the head, and the 

 two processes are, indeed, very much alike. 



As the hollow leg-discs grow out from the body in tlie 

 late larva they drag a mass of myoblasts, which lie in close 

 contact with the leg-discs throughout larval life, after them. 

 In the defaecating larva, while the legs are yet very short, 

 these have grouped themselves in each segment in opposite 

 columns, in the position they are to assume in the adult, 

 i.e., we get the rudiments of flexor and extensor muscles. 

 As in the head muscles, the myoblast columns, whose cells 

 continue to divide mitotically, grow in thickness. In the 

 pupa six hours after defaecatiou, i.e., earlier than in the 

 head muscle, the upper ends of these muscles become dragged 

 apart by the integumental cell insertions. By this means 

 cell columns, each corresponding to a single component of 

 one of the two muscles of each segment, are produced ; the 

 proximal ends are spread out, the distal insertions remain 

 together and become inserted on the tendons. The muscle 

 columns form syncytia in the usual way, which, developing 

 stria tions, transform themselves into the muscles as we see them 

 in the adult. The outer cell walls persist, of course, as the 

 sarcolemma. 



