THE NEURO-MUSCULAR SPINDLE 129 



tion at the other edge. This gives a false circulation of fluid 

 through the tissues which simulates rather closely what is seen 

 when one examines a bit of tissue with the blood still moving in 

 the capillaries. One sees the constant shifting of the cells, flat- 

 tening out and becoming rounded again as they are jostled 

 against each other by the moving fluids — the whole, a seething 

 restless mass of cells. At this stage the nerve ends are seen to 

 shift their positions among the cells, leaving old ones and com- 

 ing to lie between new ones. Later, when a union is established 

 between the nerve and myoblasts, this does not occur but they 

 simply move back and forth with the cells to which they are at- 

 tached. These preparations give one some idea of the continual 

 motion of all the cells during life, which is lost altogether in 

 studying fixed tissue only. From this it is an easy step of the 

 imagination to picture the amoeboid growing tips of blood 

 capillaries, lymphatics, and nerves working their way in and out 

 among the tissue cells. 



The first change of consequence from this very primitive pic- 

 ture is in the 15 mm. stage. Here, while the essential picture is 

 the same as in a 12 mm. embryo, still one can see a decided 

 advance. The nerve fibers are beginning to form a loose and 

 very wide-meshed plexus throughout the muscle mass; the ter- 

 minal fibrils are more numerous. These delicate tendril-like ends 

 have come into connection with some of the myoblasts (fig. 3). 

 This connection is a rather simple affair. The terminal branches 

 either wrap themselves around the cell or widen out into small 

 circular nets upon the surface as they cross the cell. As yet, 

 not all of the terminal fibrils have united with myoblasts. So 

 far I cannot say absolutely whether the endings are sensory or 

 motor. Even here, however, there is a certain amount of differ- 

 ence in the type of terminal network from that in the motor end- 

 ings. The spindle is already more complex than even later 

 stages in the development of the motor plates. In the embryos 

 that are longer than 15 mm. there is no fficulty whatever as 

 can be seen readily by comparing the spindles with the figures 

 of the motor endings, which I have included for this purpose. 



