138 WILLIAM F. ALLEN 



tribiiting to the formation of the ribbon-Hke spinal cord of 

 Cyclostomes. In this paper the questions of chief interest are 

 concerned ^\dth the manner and cause for the union of the motor 

 and sensory components of the spinal nerves in vertebrates 

 above Amphioxus and Petromyzon, the appearance of muscle 

 sense endings in the specialized M. cordis caudalis outside the 

 caudal heart, and the presence and possible significance of certain 

 peripheral ganglion cells found along the course of the vagus, 

 glossopharyngeal and some of the spinal nerves. 



Material and methods 



The material for this paper consisted of several sets of serial 

 sections taken from various levels of adult Polistotrema (Bdello- 

 stoma) and Amphioxus; together with serial sections of a num- 

 ber of Polistotrema, Squalus acanthias, turtle, pigeon and pig 

 embryos. I am indebted to Prof. R. E. Scammon for the use 

 of his very complete serial collection of Squalus embryos. The 

 results were obtained largely from a study of graphic reconstruc- 

 tions which were prepared after the usual method. Careful 

 projection drawings were made of each section. These were 

 later checked up for errors using a higher magnification. A 

 common base or projection line was drawn horizontally across 

 the ventral surface of the notochord in the first drawing from 

 which all structures were measured off and plotted on milli- 

 meter paper. In order that some of the minor details of descrip- 

 tion might be eliminated from the text, very complete and de- 

 tailed descriptions of the figures have been given at the end of 

 this paper to which the reader's attention is directed. 



MANNER OF DISTRIBUTION OF THE MOTOR AND SENSORY SPINAL 



NERVE FIBERS IN ADULTS AND EMBRYOS OF VARIOUS 



VERTEBRATES 



1. Comparison of the two extremes, Amphioxus and Petromyzon, 



with mammals 



An examination of figures 29 and 30 discloses the well-known 

 differences between the spinal nerves of Amphioxus and a 



