170 WILLIAM F. ALLEN 



rate, the sensory rami followed the intermuscular septa of the 

 myotomes and the motor rami penetrated the inner surface of 

 the myotomes. In Pohstotrema it was e\ddent that the ven- 

 tral motor rami joined their corresponding sensory rami, ir- 

 respective of whether the motor rami were situated cephalad or 

 caudad of their respective sensory rami, in all cases where such 

 union takes place. In early Squalus embryos it was shown that 

 the ventral motor rami were formed first and that there was a 

 tendency for the ventral prolongations of the neural crest to 

 migrate cephalad toward their corresponding motor rami. 

 Also the sensory fibers for the dorsal rami left the caudal surface 

 of their respective ganglia at exactly the point of crossing of 

 their corresponding motor rami. 



In reply to the question what causes bring about the union 

 of the original separate motor and sensory spinal nerves into a 

 common mixed trunk, it can be said that there are probably 

 several factors involved in producing this result, some of which 

 are mechanical, due to the mode of development of neighboring 

 structures, while others may be attributed to a possible chemical 

 attraction, chemotropism. 



1. Mechanical factors. 



The most obvious is doubtless due to the rapid and continuous 

 growth of the myotomes, which is much more pronounced than 

 that of the skeletal or neural axis. The general effect of this 

 rapid growth of the myotomes is to crowd the muscle plates 

 backward, which is most noticeable in the more caudal seg- 

 ments and more pronounced in the dorsal and ventral portions 

 than the central portion of any given muscle plate. As a result 

 of the general shifting backward of the muscle plates the spinal 

 nerves have been carried along with them. In this connection 

 the following points in the development of the spinal nerves 

 should be recalled : (a) At first the motor components of a spinal 

 nerve are more closely connected with the myotomes than the 

 sensory, (b) In most cases the motor rami are formed before 

 the sensory, (c) The motor components of a spinal nerve are 



