122 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch. 



lateral halves and then grows outwards on each side opposite each 

 myotome, each outgrowth representing a single spinal ganglion. 

 Eventually these break apart but in some of the more primitive 

 Vertebrates the intervening portions of neural crest persist for a 

 time in the form of a distinct longitudinal commissure (Fig. 68) 

 linking up the series of spinal ganglia to one another (Elasmobranchii 

 — Balfour : Dipnoi). 



The mode of development of the fibres forming the dorsal root, 

 whether by outgrowth from the ganglion-cells of the spinal ganglion 

 or by differentiation of an already existing protoplasmic bridge, comes 



ect 



eel. 



s.c. 



ect. 



s.c. 



Fig. 67. — Illustrating the mode of origin of the spinal ganglia. 



A, fowl embryo with four mesoderm segments (after Neumayr, 1906) ; B and C, Torpedo 4 mm. 

 embryo (after Dohrn, 1902) ; ect, ectoderm ; g, rudiment of ganglion ; s.c, spinal cord. 



under the general controversy as to nerve-development and need not 

 be specially discussed. 



Cranial Nerves. — The development of the cranial nerves has 

 been investigated by many workers and an immense amount of 

 detailed observation has been accumulated. There is however great 

 discrepancy in detail between the results obtained by different 

 workers, and much of the observation seems to be perilously near 

 the limit of probable error. Consequently the material seems hardly 

 ripe for treatment in a text-book of a general kind and nothing of 

 the sort will be attempted here beyond noting one or two points of 

 particular importance. 1 



1 A modern account of the development of cranial nerves will be found in 

 Neumayr (1906). 



