338 DR. A. MILNES MARSHALT-. 



in tlie median ventral line, tlie section passing througli'the 

 anterior part of the oral aperture. 



In figs. 34 and 35 the same structure is shown in longi- 

 tudinal and vertical section; fig. So, which is the more 

 superficial of the two,^ shows the pharynx (al'.J, with the 

 branchial arches and the anterior continuation of the buccal 

 cavity {al'.) Fig. 34, which passes through the root of origin 

 of the olfactory nerve, and therefore, as is evident from fig. 

 33, very close to the median line, passes also through the 

 mouth; it shows very clearly the manner in which this 

 anterior prolongation {al'} extends forwards in front of the 

 anterior margin of the mouth, 



I have unfortunately not yet succeeded in tracing the 

 development of this prolongation, and do not even know for 

 certain whether it appears before or after the formation of 

 the mouth, or whether it is lined by hypoblast or epiblast. 



At a stage a little later than that just described, when the 

 growth of the anterior part of the head has carried the 

 nose considerably farther forwards, this prolongation exists 

 in the form of a pair of ccecal diverticula, stretching for- 

 wards from the anterior part of the buccal cavity towards 

 the olfactory pits. These are well shown in fig. 32, a trans- 

 verse section through the anterior part of the head of a 

 salmon embryo about a week after hatching. The section 

 passes through the extreme anterior end of the forebrain 

 {f. b.) in front of the 'origin of the olfactory nerves, through 

 the two eyes (o. c), the superior recti muscles (r. 5.), the trabe- 

 cular plate (tt\), the hinder end of the two olfac tory pits {olf.), 

 and the diverticula of the buccal cavity (aV.) close to their 

 anterior terminations. At a stage a little later still, these 

 diverticula appear to shrink and disappear; at least I 

 have failed to recognize them in sections. 



Whatever these diverticula may prove to be, their exist- 

 ence is certainly of some interest in connection with the 

 visceral-cleft theory of the olfactory organ ; they show, at 

 any rate, that there do exist diverticula of the alimentary 

 canal towards the olfactory organs ; they may possibly be 

 taken as indications of a former extension forwards of the 

 alitnentary canal to the anterior end of the head ; while their 

 paired condition, shown in fig. 32, may perhaps be an indica- 

 tion of relationship to the paired lateral diverticula of the 

 alimentary canal, which form the rudiments of the hinder 

 visceral clefts. 



Again, if the olfactory organs are gill clefts and the Schnei- 

 derian folds gills, not only must these clefts have originally 

 communicated with the buccal cavity, but the vertebrate 



