THE NEURO-MUSCULAR SPINDLE 137 



In the 150 mm. stage, figure 10 shows how the nerve at times 

 divides into several smaller branches as it approaches a fiber, 

 some of them going to one end, some to the other, but all re- 

 uniting in a common network upon the muscle fiber. Now the 

 placques are nearly all arranged perpendicular to the long axis 

 of the fiber. All of them are the basis of the most delicate rami- 

 fications with the usual deeply staining nodules, along the fibrils 

 and at their points of branching, only here much smaller than 

 those on the axones themselves. These placques are applied 

 closely to the under side of the sarcolemma, which is beginning to 

 be formed around the muscle cell at this stage of development. 

 They usually extend about one-half to two-thirds of the way 

 around the muscle fiber, rarely encircling it completely. They 

 appear to be made up of very fine granules with very high magni- 

 fications. In the later stages (fig. 11) from 175 mm. embryos on 

 up to the full-term pig, they undergo a decided change in appear- 

 ance. Instead of being the large structures with the delicate 

 neurofibril network upon them which at once catch the eye, 

 they become rather inconspicuous. The axones have thickened 

 to a marked extent and in some cases have straightened out con- 

 siderably, forming a close spiral around the fiber, still connected, 

 however, by the rest of the original delicate plexus. At inter- 

 vals along these thickened axones, though considerably hidden, 

 are these same placques. In other cases where the spiral is not so 

 prominent the light granulation of the placques is obscured by 

 what seems to be a continuation or spreading out of the axone, 

 undoubtedly due to an increase in the neurofibri net upon the 

 placques to such a point that the methylene blue no longer 

 differentiates them. These correspond with the type seen in the 

 adult endings in tendons. Dogiel in his article on "The fibrillar 

 structure of tendon spindles," refers to them thus: 



Some of the fine branches spread out as three or many cornered 

 pieces and from these corners other delicate branches extend. The 

 presence of these larger pieces gives the nerve apparatus an especially 

 characteristic appearance. They have the form of placques which 

 seem to grasp the fibers. The fine branches contain neurofibrils which 

 in the placques widen out into a net from the corners of which they then 



