THE NEURO-MUSCULAR SPINDLE 131 



as there is between the violets. In some few cases it takes the 

 stain very deeply, which also lessens the contrast of colors. This 

 is especially true in the later stages as we approach the adult 

 type. Here, however, it is obscured by the thickening of the 

 neurofibrils plexus upon it, which if overstained will be a solid 

 mass inseparable from the branching axone. I shall have more 

 to say later as to the exact structure of this placque, which I 

 regard as an intermediate substance between the neuro- and myo- 

 fibril. 



By the time the embryo is 25 mm. long, the endings have 

 taken on a little more definite form. They are more numerous 

 than earlier. Instead of the endings being bunched upon small 

 round cells — as the myoblasts have grown into spindle cells— 

 they are now spread out to a greater extent (fig. 5) . The axone 

 forms a very delicate plexus enclosing a part of the cell in a 

 complete net. While the placques still lie more or less at ran- 

 dom upon the muscle cell, they show a tendency to lie at right 

 angles to the long axis of the cell. The number of muscle cells 

 included in a single ending varies from one to four or five. More 

 cells are added, from time to time, as the embryo develops. 

 There does not seem to be any definite rule in this regard. Even 

 in the adult neuro-muscular spindles the number of intrafusal 

 fibers varies between wide limits. Adult spindles with only one 

 or two fibers, however, are rare. 



By the 55 mm. stage there remain few, if any, free nerve 

 fibrils. All that I followed out led to either sensory or motor 

 endings in the muscle. The axones intertwine among the 

 muscle fibers, forming a very intricate plexus so arranged as to 

 bring almost all of the muscle fibers into contact with a nerve. 

 The nerves run along rather loosely in the smaller meshes of 

 the plexus, leaving it abruptly from time to time, to extend off 

 and end upon some nearby muscle fibers. The axones in the 

 ending wind around the intrafusal fibers in a loose network. 

 Some end upon definite placques, which usually occupy the 

 middle section of the spindle, while others extend along the 

 fibers for some distance and end in flower-spray ramifications. 

 Within the spindles the muscle nuclei are more numerous than 



