328 



DR. A. MILNES MARSHALL. 



{Mx.) and mandibular (Mn.) arches. Behind the mandibular 

 arch, between it and the hyoidean arch, is the cleft, the two 

 sides of which are supplied by the facial nerve. Between 

 the buccal cleft and the olfactory pit a cleft intervenes — the 

 lachrymal cleft; so that the number of clefts in the trout 

 agrees completely with that we have already found in the 

 chick. 



The resemblances between the olfactory pit and the 

 visceral clefts are, however, not simply those of general 

 appearance and relations ; they are of a far more intimate 

 nature, and extend even into the details of histological 

 structure. For studying these more intricate relationships 

 the dogfish has proved the most suitable. 



The olfactory organ of a dogfish does not long remain 

 a simple pit; very soon after its first appearance its walls 

 become thrown into a series of folds — the rudiments of the 

 Schneiderian folds of the adult. I wish here to call atten- 

 tion to the resemblances between these folds and the series 

 of folds which, arising from the sides of the visceral clefts, 

 form the rudiments of the gills. 



I have not myself observed the presence of the rudimentary 

 Schneiderian folds in embryos younger than stage M, but 

 Balfour has shown that they not only exist, but have acquired 

 the characteristic adult arrangement in embryos *' a little 

 older than k."i With regard to the gills, Balfour's descrip- 

 tion is as follows : — " Towards the close of stage k there 

 arise, from the walls of the second, third, and fourth clefts, 

 very small knob-like processes, the rudiments of the external 

 gills. These outgrowths are formed both by the lining of 

 the gill cleft and by the adjoining mesoblast."^ If, indeed, 

 the times of appearance be not absolutely identical in the 

 two cases, the correspondence is, at any rate, sufficiently 

 striking. 



Fig. 23 is a horizontal and longitudinal section through 

 the head of a dogfish embryo at stage n, magnified twenty 

 diameters ; it passes through the fore and hind brains, the 

 notochord (w), the eyes (o. c), the oculo motor (iii) and tri- 

 geminal (y) nerves, and through the olfactory pits (olf.). 

 The bottoms of these j)its are seen to be thrown into a series 

 of small equidistant folds — the Schneiderian folds. 



Fig. 24 is a transverse section through the body of the 

 same embryo, taken a short way behind the head, and pass- 

 ing through one of the branchial arches on either side. The 

 section which, like the preceding one, is magnified twenty 



' Op. cit., p. 184, and Plate XIV, fig. 14. 

 - Op. cit., p. 211. 



