Morphology of the vertebrate olfactory organ. 305 



and olfactory pit appears to be acquired at least as early as 

 stage K. 



h. In the chick. — I propose to consider the chick next, 

 partly because, having devoted more time to the embryology 

 of the chick than of other vertebrates, I have a better and 

 more complete series of preparations to refer to, and partly 

 because I wish to direct particular attention to the very close 

 correspondence that exists between the chick and the dog- 

 fish in the mode of development of the olfactory nerve. 



Concerning the early stages of the olfactory nerve in the 

 chick T have little or nothing to add to the account I have 

 already given in this Journal.^ The result of a careful 

 re-examination of my former preparations, and the investiga- 

 tion of a considerable number of new specimens prepared 

 since my former paper was published, has been to confirm 

 my previous description on all points. Though I have again 

 failed to trace satisfactorily the changes that occur between 

 the thirtieth and fiftieth hours my further work has shown 

 no reason for altering the view I have previously expressed, 

 that the olfactory nerve is developed, like all the other 

 cranial nerves (except the optic, the sixth, and (?) the fourth 

 nerve), from the tieural crest^. However, whether this be 

 so or not is of comparatively little importance to the subject 

 with which we are now concerned. 



Plate XIII, fig. 10, represents part of a transverse section 

 through the fore part of the head of a duck embryo towards 

 the end of the fourth day. This figure, which is repeated 

 with slight alterations from a former paper,^ happens to show 

 the points to which I wish to call attention rather better 

 than any of my chick preparations, the specimen from which 



1 ' Quarterly Journal of Microscopical Science/ January 1S78, pp. 17 — 23. 

 To avoid repetition, I beg to refer the reader to the detailed account of the 

 early stages contained in this paper. 



- I take this opportunity to make a slight alteration in the nomencla- 

 ture adopted in my former paper. I have there suggested the term 

 neural ridge for the longitudinal ridge of cells which grows out from the re- 

 entering angle between the external epiblast and the neural canal, and from 

 which the nerves, whether cranial or spinal, arise. Since this ridge appears 

 before closure of the neural canal is effected, there are manifestly two 

 neural ridges, one on either side ; but I have also applied the same term, 

 neural ridge, to the single outgrowth formed by the fusion of the neural 

 ridges of the two sides after complete closure of the neural canal is effected, 

 and after the external epiblast has become completely separated from the 

 neural canal. I propose in future to speak of this single median outgrowth 

 as the neural crest, limiting the term neural ridge to the former acceptation. 

 Thus, while there are two neural ridges, there is only one neural crest, a 

 distinction that will be at once evident on reference to my former figures. 



^ ' Journal of Anatomy and Physiology,' vol. xi, plate xxi, fig. 13. 



