122 EMBRYOLOGY OF THE LOWER VERTEBRATES 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 tin- scries of spinal ganglia to one another (Elasruobranchii 

 1 lalfour : 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 



eel 



FIG. 67. Illustrating the mode of origin of the spinal ganglia. 



A, fowl embryo with four mi-sodi-rni s.-mi-nts (aft IT Ni-uinavr, I'.'Dti); \\ and (', 'l',>r/;l,> -1 mm. 

 i-mbryo (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. >s ^'v/\ 



CRANIAL NERVES. The development of the cranial nerves has 



iiriMi investigated by many workers and an immense amount of 



detailed observation has been accumulated. There is however pvat 



"paney in detail between the results obtained by different 



workers, ;nd much .f the observation seems t< be perilously near 



the limit of probable error. Consequently the material seems hardly 



ripe for treat incut in a text-book of a gcnei-al kind and not liin^ 'f 



rt will br atlriiiplcd hdv beyond nolin^ one or tw<> points of 



parti(-nlar importance.' 



1 A modern account of the development of cranial nerves \\ill In- round in 

 .. yr (1906). 



