3.24 DR. A. MILNES MAHSHALL. 



The vertebrate olfactory organs make their first appear- 

 ance as '' a pair of slight thickenings of the external epiblast 

 on the under surface of the forebrain, immediately in front 

 of the mouth. . . . Each thickened patch of skin soon 

 becomes involuted as a shallow pit."^ 



In the dogfish these thickenings appear " during a stage 

 intermediate between L and k" (Balfour). Their condition 

 during stage k is well shown in figs. 13 and 14 {olf.) ; the 

 former figure being a longitudinal and horizontal section, 

 the latter a vertical and transverse one. The exceedingly 

 close proximity of the bottom of the olfactory pit to the 

 brain is well shown by both figures. Fig. 13 shows also 

 that at a time when the nose is in a very rudimentary con- 

 dition, the eye (o. c.) has already made considerable progress 

 in development, a point to which Balfour has already directed 

 attention. 



The communication between the visceral clefts and the 

 exterior is established almost simultaneously with the first 

 appearance of the olfactory pits. At stage L there are 

 " three visceral clefts, none of which are as yet open to the 

 exterior."" At stage k, according to Balfour, '^^ four visceral 

 clefts are now visible, all of which are open to the exterior, 

 but in a transparent embryo one more, not open to the 

 exterior, wotild have been visible behind the last of these. "^ 

 The visceral clefts, then, first become open to the exterior 

 between stages L and k, and we have already seen that it 

 is between these same two stages that the thickenings of the 

 epiblast appear which form the earliest rudiments of the 

 olfactory pits. 



In the chick the early stages of development of the olfac- 

 tory pits closely resemble those just described in the dogfish. 

 Fig. 1 represents a longitudinal and horizontal section 

 through the head of a fifty-four hours' chick embryo ; the 

 section, which may with advantage be compared with fig. 13, 

 shows on the right side the olfactory pit (o//".), formed by the 

 thickened and involuted epiblast, and in close proximity 

 to the forebrain (f. h.) ; on the left side the section, which 

 is a little oblique, passes through the thickened epiblast 

 forming the margin of the olfactory pit, and through the eye 

 (o, c). Two visceral clefts {v. c.) are shown, both open to the 

 exterior. 



The earliest period in the chick at which I have noticed 

 the thickening of the olfactory epithelium is about the 



^ Balfour, op. cit., p. 184. 



2 Balfour, op. cit., p. 77. 



3 Qp cit., p. 78. 



