221 



3) This mass of cells is the rudiment of the ganglion of 

 a posterior root of a cranial nerve'. 



For some time cells continue to be given off form the thickened 

 epiblast — , and of those already given off many show nuclear figures, 

 indicating rapid division. 



Thus in the end a fair sized ganglion is formed, but it is still inti- 

 mately connected with the skin. 



4) But this is notali; the portion of thickened epiblast 

 at Avhich a ganglion is formed is also the point of origin 

 of one of the segmental sense organs. 



In fine, the first segmental sense organs ofthe head and 

 the cranial ganglia are formed in intimate connection. 



The importance of this fact cannot be overestimated. It shows so 

 close a relationship between the segmental sense organs and segmental 

 ganglia of the head that in all future work dealing with the posterior 

 roots of cranial nerves this relationship will have to be taken into 

 account. 



It further justifies the endeavours which have been made (Eisig) 

 to attach morphological importance to the segmental sense organs. 



Moreover I think the time is past, if it ever existed since Ley- 

 dig's brilliant researches, when zoologists can with any pretence to 

 morphological accuracy and meaning speak of these important sense 

 organs — viz. the segmental sense organs, — as mucous canals. 



5) At a later stage the ganglion separates from the skin, 

 only remaining connected with its segmental sense organ 

 by a small nerve branch, which also in all cases is split off 

 from the skin. 



The mode of origin discribed above holds for the ganglia of the 

 following segmental nerves, fifth, seventh, eighth, ninth and tenth, and 

 also for the ciliary ganglion. I leave out of question for the present 

 what is the posterior nerve root of the latter ganglion. 



I have not yet quite satisfied myself that the olfactory ganglion 

 arises in this way, but my own researches, incline me towards the 

 affirmative, an inclination which is much strengthened by careful 

 perusal and examination of Prof. Marshall's paper and figures on 

 the Morphology of the Olfactory organ -. 



1 Mr. Spencer has quite independently discovered this mode of origin of cra- 

 nial ganglia in Amphibia. In point of time perhaps even before me. — It is very 

 probable that such a mode of origin occurs in Teleostei. Indeed, I was led to suspect 

 • it in Salmo some two years ago. 



'- Marshall, The Morphologv of the Vertebrate Olfactory organ. Quart. 

 Journ. Microsc. Sci. Vol. XIX. 1879. p. 300. 



