186 WILLIAM F. ALLEN 



supporting tissue. It is possible that such a stage exists and 

 is passed over so rapidly that additional material would be 

 required to demonstrate it. 



8. In all embryos where the motor and sensory elements of a 

 spinal nerve formed mixed nerves, this union begins peripherally 

 and proceeds centrally. In Polistotrema the motor compo- 

 nents always join the sensory, irrespective of the position of the 

 nerves. 



9. The most obvious of the mechanical factors in the growth 

 of the embryo which might contribute to the approximation of 

 the motor and sensory spinal nerves is the more rapid and longer 

 continued growth of the myotomes over the skeletal and neural 

 axes. Suice the motor components of an embryonic spinal 

 nerve are situated in front of the sensory and are formed earlier, 

 they would be subjected longer to the same force of the general 

 shifting backward of the myotomes, and would in consequence 

 be carried further caudad than the sensory nerves. 



10. The possibility of the existence of a chemical attraction 

 (chemotropism) between the embryonic motor and sensory 

 spinal nerves was suggested from two observations. First, the 

 reconstruction of several of the caudal spinal nerves of the 7.5 

 mm. Squalus embryo demonstrated that the ventral migration 

 of the spinal ganglion cells and fibers to form the ventral sensory 

 rami and the vertebral sympathetic system, assumed a cephalic 

 direction in passing between the corresponding motor compo- 

 nents, as if they were attracted by the more cephalic motor rami. 

 Second, the dorsal sensory rami fibers of Squalus did not grow 

 out of the spinal ganglia until some little time after the corre- 

 sponding dorsal motor rami fibers had migi-ated caudally across 

 the lateral surface of their ganglia, and when they finally ap- 

 peared, it was from the caudal surface of their ganglia at the 

 point of crossing of the dorsal motor rami, with which they 

 later join in forming mixed rami dorsales. 



11. Isolated or clustered nerve cells were found along the 

 course of certain of the sensory and mixed spinal rami of Polis- 

 totrema. In the extreme caudal sensory nerves these cells were 

 sufficiently numerous and scattered throughout the peripheral 



