No. 2.] 



LIXE SYSTEM OF BATRACHUS TAU. 



J43 



to the formation of these branches. According to Hoffmann 

 (1 8), the lateral nerve in Salmo arises from a string of cells in 

 the nervous layer of the ectoderm some time previous to the 

 development of the organs. This string gradually moves out 

 of the ectoderm, coming to lie at some distance internal to it, 

 but connected at intervals 



<S5 



///-.- 



by short side branchlets with 

 the locality where the future 

 segmental sense organs are 

 to arise. Hoffmann's (i8) 

 observations were made on 

 embryos of a teleost, which 

 he regards as a less favor- 

 able form than the selachian, 

 in which, according to Sem- 

 per (22), it is uncommonly 

 easy to show the origin of 

 the lateral nerve. 



In the section of the 

 Acanthias embryo (Fig. 14), 

 there is an evident exten- 

 sion of nerve fibres from 

 the vagus ganglion accom- 

 panying the lateral line 

 rudiment. In cross-section 

 these fibres are seen to con- 

 stitute at this stage a part 

 of the lateral line (Cut 4). It will be necessary to study the 

 changes taking place in the later stages before a final conclusion 

 can be drawn, but it would appear that the thickened ectoderm 

 forming the lateral line and the extension of the outgrowths of 

 the ganglion cells were associated during the early history 

 of the structure. 



Hoffmann's (18) description of the origin of the nerve 

 becomes more intelligible after the study of the selachian 

 embryo, although in both the case of Salmo and Acanthias the 

 exact mode of origin of the sense organs remains uncertain. In 

 Batrachus, on the other hand, the origin of the sense organs 



Cut 4. 



- Cross-section of lateral line of 

 Acantllias embryo. 



