58 H. V. NEAL 



the ciliary — in their adult structure, appear to be sympathetic. 

 Their derivation from the neural crest favors the inference that 

 the sympathetic anlagen of the trunk have a similar origin. 



It may be further urged in favor of the ganglionic origin and 

 against the medullary derivation of the sympathetic anlagen in 

 Squalus that the medullary migration into somatic motor nerves, 

 like the abducens and the hypoglossus which have no sympa- 

 thetic anlagen, is as striking as in those nerves associated with 

 sympathetic ganglia. The migrant cells of the hypoglossus and 

 abducens can have no other destination than the neurilemma. 

 On the other hand, sympathetic cells do migrate along the vagus 

 nerve, the ganglionic nerve of the hypoglossus segments. So 

 the evidence seems to favor the opinion that in Squalus the 

 sympathetic anlagen are derived from neural crest elements and 

 not from emigrated medullary cells. 



11. Summary of the histogenesis of spinal somatic motor nerves 



The more important features of the histogenesis of spinal so- 

 matic motor nerves in Squalus are shown in figures 23 to 30, 

 and may be briefly summarized as follows: 



Nerve and muscle are not primarily connected with each other 

 in Squalus embryos. Previous to the establishment of proto- 

 plasmic connection, the space normally found between somite 

 and neural tube is filled by a vacuolated, non-staining, non- 

 protoplasmic liquid containing a relatively small amount of coag- 

 ulable material. 



Protoplasmic connection of somite and tube is established by 

 an amoeboid protoplasmic extrusion from cells in the ventro- 

 lateral wall of the neural tube forming the 'protoplasmic bridges' 

 of Paton ('07j or 'plasmodesms' of Held ('06). The cell proc- 

 esses which form these connections extend gradually along the 

 median surface of the somite between myotome and sclerotome. 



Within these processes the neurofibrils soon make their appear- 

 ance, as shown in Cajal and Bielchowsky-Paton preparations. 

 This evidence demonstrates the neuroblastic nature of the cells 



