250 ALICE JOHNSON AND LILIAN SHELDON. 



fusion found by Froriep in Mammals (7). Mr. Beard considers 

 that, in EiasmobrancliSj all the maia branches of the nerve 

 except the post-branchial and the part between the ganglion and 

 the brain are split off from the epiblast. Van Wijhe holds 

 that the epiblast takes some share in the formation of the 

 ganglion at least, while Froriep expresses doubt as to this 

 pointj comparing the fusion to the similar fusion of nerve-cells 

 and epithelium cells in the ear. We are strongly inclined to 

 the last view. Professor Marshall (16) has shown how very 

 early the nerve-cells of the ear become indistinguishably fused 

 with it, and there seems no reason why this should not be the 

 case with other sense organs. As to the splitting off of the 

 nerve-trunks from the skin, Mr. Beard's observations and 

 deductions seem to us inconclusive. 



In Elasmobranchs Professor Balfour mentioned and figured a 

 fusion between the mucous canals of the head and the nerves 

 supplying them, no line of demarcation existing between the 

 two structures (v. loc. cit., pp. 144, 145, plate xii, fig. 7). He 

 describes this as occurring first in his Stage P, but it is possible 

 that it may take place rather earlier in the Elasmobranchs, as 

 it certainly does in the Newt. Mr. Beard seems to have de- 

 tected the earlier fusion in Elasmobranchs, and to be unaware 

 that the fact of the fusion was described by Professor Balfour, 

 who found that the nerves were all derived from the brain out- 

 growths, as we believe to be the case in the Newt. It appears 

 to us that the epiblast in this animal takes no part in the 

 formation of the ganglion or nerve branches, and that the 

 special nerve to the sense organ is an outgrowth from the 

 ganglion, advancing pari passu with the withdrawal of the 

 latter from the surface, so that there is at no time any break 

 in the connection between the sense organ and its nerve supply. 

 The withdrawal of the ganglion and formation of the nerve 

 is only a result of the differentiation of the nerve supply into a 

 ganglionic and a fibrous part. 



The disposition of these sense organs seems to us a very 

 insecure guide to the segmentation of the head. Mr. Beard 

 considers that the relations of the sense organs to the gill- 



