168 WILLIAM F. ALLEN 



5. Arrangement in the turtle and pigeon embryos 



From an examination of a reconstruction of a 6 and an 8 mm. 

 pigeon (figs. 31 and 32), a transverse section through a spinal 

 nerve of a 10 mm. turtle (fig. 33), and from series of chicks 

 and pigs it is apparent that the various motor and sensory 

 components of a spinal nerve lie in the same vertical plane. 

 None of the motor elements were found to be entirely sepa- 

 rated from the sensory as in Polistotrema and Squalus embryos. 

 It is possible that this stage is skipped in the higher vertebrates 

 or it may have been passed through so rapidly that it was missed 

 in my series. The latter view is supported by the fact that, 

 while the motor and sensory rami come in contact, they are 

 separated in so far as any exchange of fibers is concerned for 

 some distance peripherally, as is indicated by a difference in 

 structure, the sensory portion consisting mainly of cells and the 

 motor portion chiefly of fibers. Inasmuch as a study of the 

 development of the spinal nerves in Anmiota can be of little 

 assistance in solving how the motor and sensory rami unite, a 

 detailed description of the above mentioned reconstructions is 

 unwarranted. It can be mentioned, however, that the various 

 rami of a spinal nerve have apparently a slightly different order 

 of appearance from cyclostomes and selachians. It will be 

 seen from the 6 mm. pigeon reconstruction (fig. 31) that the 

 sensory rami ventrales are well-formed before there is any 

 trace of motor rami dorsales. The latter are shown in the 8 

 mm. pigeon (fig. 32) to be formed about the same time as the 

 sensory rami dorsales. All of the peripheral sensory fibers leave 

 the ventral pole of their respective ganglia to separate imme- 

 diately into dorsal and ventral rami. 



POSSIBLE CAUSES FOR THE UNION OF THE MOTOR AND SENSORY 

 SPINAL RAMI IN ALL VERTEBRATES ABOVE THE CYCLOSTOMES 



From the previous section it is clear that in the simplest 

 vertebrates, Amphioxus and Petromyzon, the motor and sen- 

 sory components of a spinal nerve always remain separated, 

 and in the higher vertebrates from selachians up the sensory 

 and motor elements intermingle in embryonic life, forming mixed 



