206 JOHN BEARD. 



brain on the other. In fact, these sense organs may very well be 

 regarded as special sense organs of the gill-clefts or as branchial sense 

 organs. This conclusion Prof. Froriep and I have independently 

 arrived at. 



From the above considerations, and from the facts of development 

 recorded here, it also follows that the ganglia of the posterior roots are 

 primitively ganglia of these branchial sense organs. Originally con- 

 nected directly with this branchial sense organ, the ganglion of the 

 posterior root has now left its primitive position and has come to lie in 

 the mesoblast, being only connected with its sense organ by nerve- 

 fibres. In this conclusion as to the nature of the ganglion I am again 

 independently in agreement with Froriep and Spencer. 



In describing the schematic development of a dorsal root I have, I 

 think, sufficiently emphasised its true nature. Primitively, a dorsal 

 root of a cranial nerve is the nerve of a gill-cleft, and is apparently 

 only connected with the innervation of its cleft. It sends fibres from 

 the brain to the sense organ and ganglion above the cleft, thence other 

 fibres pass to the musculature and walls of the cleft (fig. 50). 



It is not without importance to notice that any division of the dorsal 

 root of a cranial nerve into so-called dorsal and ventral branches is 

 2?rimitively absent (fig 50). Such divisions only occur in the later 

 development in consequence of the separation of the ganglion from the 

 skin, and of the formation of a greater number of branchial sense 

 organs. Of course the ventral branch is there from the start, but in 

 itself it is mainly motor and gives rise to no ganglion, and probably 

 never has sense organs in connection with it. It certainly is not 

 directly concerned in the innervation of a primitive branchial sense 

 organ. Through a misunderstanding of this point Prof. Froriep has 

 been led into rather serious errors as to the nature of the dorsal roots. 

 He concluded from Van Wijhe's researches, and, I must admit, not 

 without reason, for the matter is there very vaguely stated, that the 

 branchial sense organ and ganglion could occur on the ventral branch 

 of a cranial nerve as well as on a dorsal. This conclusion led him to 

 the opinion that the auditory nerve is a ventral branch. The blame 

 of the matter lies very much at the door of Van Wijhe, for he 

 described a cranial nerve (dorsal root) as typically possessing two 

 branches, a dorsal and a ventral one, both of which could possess a 

 ganglion. Now, we have seen in the development that the so-called 

 dorsal branch (supra-branchial nerve) forms late in the development, 



