RELATION OF THE NEMEETEA TO THE VERTEBRATA. 633 



explanation is offered as to the origin and meaning of tlie 

 two curious branches which unite respectively the ganglia of 

 the fifth and seventh and fifth and third cranial nerves ; they 

 may be regarded as persistent parts of the lateral nerve .... 

 in the head." 



In the third place extracts will be given from Beard's more 

 extensive paper.i He writes (p. 97) as an introductory state- 

 ment : " At present we are acquainted with no Invertebrate 

 nervous system which is built upon the same plan as that of 

 Vertebrates," and then passes to the results of his investiga- 

 tions, chiefly carried out on embryos of Torpedo and a 

 few other Elasmobranchs. I make the following selections 

 (p. 101): 



''At the point of fusion" (of the cephalic nerve with the 

 epiblast) " a local thickening of epiblast has previously taken 

 place. After the fusion has taken place a proliferation of some 

 of the cells composing the thickening ensues. The proliferated 

 cells form a mass of actively dividing elements still connected 



with the skin This mass of cells is the rudiment of the 



ganglion of the dorsal root." 



On p. 110 he adds: "Along with the separation of the 

 (vagus) ganglion from the skin the sensory thickening begins 

 to grow backwards along the lateral surface of the trunk. 

 This thickening is the rudiment of the so-called lateral line. 



The so-called lateral nerve is formed from the deeper 



portion of the sensory thickening That there is no 



actual growth backwards of the nerve is obvious enough." 



Recapitulating, we must acknowledge that the mode of origin 

 of the ganglia of the cephalic nerves, as described by these 

 authors, is certainly a peculiar one — a mode of development 

 sui generis. One of Beard's accompanying diagrammatic 

 figures, reproduced in Wiedersheim's second edition (1886) of 

 the ' Lehrbucli der Vergleichenden Anatomie' as woodcut 

 No. 265, moreover, shows how the position of the cephalic 

 ganglion, developing as an ectodermal proliferation, is in this 



' " Branchial Sense-Organs in Iclithyopsida," 'Quart. Journ. Micr. Sci.,' 

 IJovember, 1885, No. CI. 



