158 WILLIAM F. ALLEN 



the myotomes and a corresponding lengthening of the spinal 

 nerves. The ventral motor and sensory rami have made no 

 progress toward approaching each other. 



While it is to be regretted that the actual stages are lacking 

 when the ventral motor and sensory rami would be seen to ap- 

 proach and unite with each other, nevertheless, such a change 

 must take place in the great majority of the spinal nerves some- 

 where in embryos between 27 and 57 mm. in length. When 

 compared with the adult it was shown that each of the more 

 cephalic ventral motor rami joined the next following ventral 

 sensory ramus; that a few in the region of the cephalic end of 

 the M. cordis caudalis probably migrated cephalad, counter to 

 the course of the growdng myotomes which would tend to carry 

 them backward, and joined their more cephalic sensory rami; 

 while some seven or eight of the most caudal ventral motor rami 

 not only grew cephalad against the force of the growing myo- 

 tomes, but continued to grow some distance cephalad of their 

 corresponding sensory rami to end in the myotomes. I am un- 

 able to offer a satisfactory explanation why the motor rami of 

 the last spinal nerves should grow past their more cephalic 

 sensory rami to terminate in the myotomes in front, unless per- 

 chance it can be attributed to the myotomes of this particular 

 region offering a particularly strong chemotropism for these 

 motor rami. It cannot be due to their additional connections 

 with the M. cordis caudalis, for a number of the motor rami 

 behind this muscle have an identical distribution. 



Jf.. Adult and embryonic conditions in Squalus acanthias 



Since my Polistotrema material did not permit of a com- 

 plete analysis of the mode of union of the ventral motor and 

 sensory rami, it seemed advisable for this and other reasons to 

 investigate the origin of the various spinal nerves in some sela- 

 chian. A dissection was made of some of the lower abdominal 

 spinal nerves of a fetal shark at birth to obtain an adequate 

 understanding of the various motor and sensory components of 

 an adult spinal nerve, with which embryonic comparisons could 

 be made. The following description is taken from this dissection. 



