220 

 4. On the cranial ganglia and segmental sense organs of Fishes. 



By Dr. John Beard, 

 Berkeley Fellow of Owens College, Victoria University, Manchester. 



eingeg. 25. Februar 1885. 



In the following lines a brief account is given of some new facts 

 as to the development of the cranial ganglia and segmental sense or- 

 gans, and also as to the relations of the cranial ganglia to segmental 

 sense organs. 



The justification for a second preliminary note on the segmental 

 sense organs I find in the fact that my friend Mr. W. Baldwin Spencer, 

 B. A. Scholar of Exeter College, Oxford, recently informed me that some 

 of my discoveries in Elasmobranchii exactly agreed with the results of 

 his own independent researches on the development of Amphibia. 



In point of fact we have independently discovered the mode of 

 origin of cranial ganglia which I detail below. 



Mr. Spencer having written an account of his researches for the 

 April number of the Quart. Journ. of Microsc. Sci., I now give a sum- 

 mary of my own work on the development of Torpedo ocellata merely 

 premising that the full description with figures will be published with 

 as little delay as possible. 



At the outset it must be noted that what follows only refers to the 

 true cranial nerves and their ganglia, — more correctly only to the 

 posterior roots and their ganglia, which in their mode of development 

 present more primitive features than the spinal nerves and ganglia. 



It is generally accepted that in Elasmobranchii the posterior roots 

 of the cranial nerves like those of the spinal, arise as outgrowths of a 

 ridge of cells, which lies on each side of the dorsal edge of the neural 

 canal, — this is the neural ridge of Marshall. The neural ridge lies 

 \\\ the angle between the neural canal and the external epiblast, just 

 beneath and in close connection with the latter. 



Local outgrowths of cells of this ridge grow outwards and down- 

 wards, towards the lateral surface of the body and form the rudiments 

 of the posterior roots. In their course they lie directly under the epi- 

 blast. All this is well known. But I find that the subsequent events 

 are as follows. 



1) When these outgrowths reach the level of the nolochord they 

 fuse with the epiblast. 



2) At this point of fusion of any one cranial nerve with the skin 

 a local thickening of the epiblast takes place. On this a proliferation 

 of some of the cells composing the thickening ensues. The proliferat- 

 ed cells form a mass made up of actively dividing elements still con- 

 nected with the skin and fused with the posterior root. 



