THE NEURO-MUSCULAR SPINDLE 127 



With this preUminary summary of the development of the 

 muscle, which I have taken largely from Lewis (Keibel and Mall) 

 and Bardeen, we are ready to go ahead with a description of the 

 sensory nerve endings in the muscle. My own actual method of 

 procedure at first was, in general, to start with the adult endings in 

 adult muscle and work backward — gradually using smaller and 

 smaller embryos — as this kept me from falling into the possible 

 error of mistaking other structures for nerve endings, or, what is 

 more likely, other types of nerve endings for early neuro-muscular 

 spindles. Thus I was working from known to unknown. Be- 

 sides, this method had the advantage of proceeding from the easier 

 to the more difficult work, since the mechanical difficulties of 

 demonstrating the endings increases as the pigs get smaller be- 

 cause of the smaller vessels to insert the cannula into and the 

 difficulty in isolating the eye muscles from the surrounding tissue. 

 Still, for purposes of description it is more logical to begin with 

 the first appearance of the nerve in the muscle and follow it along 

 in its development toward the adult type. So I have selected 

 certain stages at more or less regular intervals, which illustrate 

 rather completely all the stages from the earliest to the adult 

 picture. In between these are many which have been omitted 

 that simply fill in the gaps and make the whole one gradual 

 process of growth. Besides, I have included one figure for each 

 stage which seems to show best the main points. One specimen 

 in this way is very inadequate, for every spindle is different in 

 pattern and size. The only way to get a really good idea is to 

 see many of them. 



The 12 mm. embryo is the earliest in which I was able to dem- 

 onstrate the nerves and feel certain that the nerve was in the 

 muscle and not simply nerve in the surrounding mesenchyme. 

 Here the muscle is composed chiefly of the large round myo- 

 blasts, among which the nerve fibers run here and there. The 

 fibers are delicate and wiry. Scattered along the axones at short 

 intervals are the densely staining nodules — perhaps the sites of 

 the future nodes of Ranvier — which give them the appearance of 

 knotted cords. These fibers (fig. 2) do not form a plexus but 

 run more or less separately, without regard to direction, in and 



