160 WILLIAM F. ALLEN 



Notwithstanding that the early developmental stages of the 

 spinal nerves of Squalus have been carefully worked out by 

 Balfour, Onodi, Hoffmann, Neal, and Scammon, it has seemed 

 best to include the early developmental history along with the 

 later. After examining a number of early Squalus series, two 

 graphic reconstructions were made of several spinal nerves of 

 the lower and middle abdominal regions of a 7.5 mm. Squalus 

 embryo. It is clear from a comparison of figure 17 with 16 that 

 the spinal nerves of Soualus present no exception to the general 

 rule, that a given segmental organ or structure tends to become 

 more embryonic in passing from a cephalic to a caudal direction. 



In figure 16 the spinal nerves of the lower abdominal region 

 will be seen to be in a very embryonic condition. It is evident 

 that the motor or effective portion of the spinal nerves show 

 considerable advancement over the sensory or receptive por- 

 tions in so far as development of fibers is concerned. Ventral 

 or motor root fibers {V.R.) together with a liberal amount of 

 cells destined to become neurilemma have grown out from the 

 ventro-lateral surface of the spinal cord, to extend laterally 

 as far as the myotomes and then to bend ventrally and follow 

 the inner surface of the myotomes for a short distance as the 

 motor rami ventrales {R.V.M.). In one spinal nerve a few 

 motor ramus dorsalis fibers {R.D.M.) branched off from a motor 

 root or motor ramus ventralis to extend dorsally a short distance 

 along the inner surface of the myotomes. The sensory or re- 

 ceptive system is represented in this figure by a continuous 

 band of neural crest cells (Neu.C.), which follows the dorso- 

 lateral surface of the spinal cord. At regular intervals the neural 

 crest ■ sends out ventral extensions or proliferations of cells. 

 They represent the anlage of the spinal ganglia and their most 

 distal portions, the beginnings of the sensory rami ventrales 

 {R.V.S.), which at first consist largely of cells. It is apparent 

 that each of these ventral prolongations of the neural crest 

 migrated ventrally between two motor roots, coming into closer 

 relationship wdth the more cephalic one and its ventral ramus. 

 In this region the spinal ganglion cells showed no peripheral 

 processes, although it is possible that silver or methylene blue 



