306 THE DEVELOPMENT OF THE VENTRAL NERVES IN SELACHII. 



are as they appear to be, the mesodermal contribution is in the end greater than the 

 ectodermal. 



I have been unable to find evidence of the participation of the emigrated 

 medullary cells in the formation of the sympathetic ganglia (Harrison :01), 

 though I am unwilling to deny this. 



c. Are the nerves ab initio connected with their terminal organs? In the sense 

 used by Van Wijhe ('89) and Hoffmann ('97) this question must be answered in the 

 affirmative. But I agree with Bardeen ('98) that the connection of the ventral nerve 

 and somite (myotome) is not at first an intimate neuromuscular attachment. The 

 end of the nerve grows freely along the median surface of the myotome. None of 

 my sections show any protoplasmic connection even of the most attenuated kind 

 between the somite and the neural tube, before the first neuraxon makes its exit 

 from the neural tube. The thick outer limiting membranes of the neural tube and 

 the somite seem to preclude a primary protoplasmic connection between nerve and 

 muscle cell. 



C. The Development of the Epi- and the Perineurium. The connective tissue 

 sheaths of ventral nerves have always been regarded as products of the mesenchyme 

 adjacent to the nerves. Miss Piatt and Goronowitsch have made it probable that much 

 of the embryonic connective tissue in the head is of ectodermal origin. This is not 

 true, however, of the connective tissue surrounding the embryonic ventral spinal 

 nerves. Most of the many layers of cells in the perineurium of Squalus are prob- 

 ably mesodermal, since they are added to the nerve from the adjacent mesenchyme. 

 But since from very early stages the nerve is surrounded by a sheath of cells of ecto- 

 dermal origin which can be traced as a continuous sheath into the adult, I am in- 

 clined to believe that for spinal ventral nerves as well as for cranial nerves (Piatt, 

 Goronowitsch) the ectoderm participates in the formation of the connective tissue 

 sheaths, epineurium, and perineurium. 



The growth of the perineurium takes place by the addition of successive layers 

 of mesenchymatous cells, resulting in the lamellated condition characteristic of the 

 adult nerve (PI. XXIV, Fig. 32). 



IV. SUMMARY. 



1. Neuroblasts and spongioblasts are undifferentiated in the early stages of 

 the ventral nerve in Squalus. 



2. The first neuraxones are formed before the migration of the cells which pro- 

 duce them. 



