154 



WILLCOX. 



from the succeeding ones only in being mucii smaller. On 

 the left the ventral root is wanting. On each side the dorsal 

 ganglion is connected by a longitudinal commissure anteriorly 

 with the ganglion of the vagus, posteriorly with the spinal 

 ganglion of the next nerve. This rudimentary nerve has dis- 

 appeared entirely, or, at most, has left only a trace in embryos 

 nine days older ; in those which have been hatched fourteen 

 days, the nerve of the second permanent segment innervates 

 also the first segment. This nerve and the one belonging to 

 the next succeeding segment leave the neural canal by the 

 same foramen, namely, the one between 

 the first neural arch and the parachordal. 

 They correspond with those considered by 

 Harrison to be the hypoglossal and the first 

 dorsal. I have not traced their distribution, 

 and am therefore unable to express an 

 opinion on this point. 





Summary. 



The cephalic region of the trout consists 



then of at least four segments. These are 



represented by more or less perfectly de- 

 Diagram representing the , , c \ • ^ A.^ r ^ ^ 



changes which take place vclopcd myotomcs of which thc first two 

 in the cephalic region of p^j^g atrophy in the course of development. 



the trout between the fifth >■ >■ -i i 



and tenth weeks of devei- The skeletal aulagc shows no trace of seg- 



opment e, auditory or- . . • <-r-'i ti • i .1 i- 



gan; m^-mv. myotomes; mcntation. Thc third scgmcnt has a rudi- 

 ni-n'-', spinal nerves ; na'- mcntary spiual ucrvc which early atrophies ; 



na^, neural arches; p, para- 1 r 1 1 



chordai; X, position of thc fourth has a typical one which three 



vagus. Shaded or dotted 



structures are permanent ; 

 those which are left light 

 disappear in the course of 

 development. 



weeks after hatching shows no sign of de- 

 generation. The condition cannot be better 

 represented than by a diagram similar to 

 that already employed by Sewertzoff. 

 This investigation was carried on in the laboratory of the 

 University of Zurich, and I gladly take this opportunity of 

 thanking Professor Lang for his kindly aid and interest, as well 

 as for the generous way in which material was placed at my 

 disposal. 



