326 DR. A. MILNES MARSHALL. 



three. Figs. 5 and 6 are drawn from consecutive sections, 

 but between figs. 4 and 5 two sections intervened. These 

 figures illustrate well the points to which I have just called 

 attention ; they show that the visceral clefts form a conti- 

 nuous series of structures, of which the most anterior is, not 

 the mouth cleft (between Mn. and 3fx.), but the cleft in 

 front of the maxillary arch ; a cleft that, following Prof. 

 Parker, T propose to speak of as the lachrymal cleft : they 

 show further that just in front of the lachrymal cleft is the 

 olfactory pit {olf.), and that the relations of the pit are such 

 as to inevitably suggest that the olfactory organ is one of 

 the same series of structures as the visceral clefts. Correct 

 the cranial flexure, and straighten out the head, and the 

 resemblance would amount almost to identity. I have only 

 to add that, though these figures are semi-diagrammatical, 

 yet as far as the outlines go, which alone concern us at 

 present, they are as absolutely accurate as I have been able 

 to make them. 



The resemblance between the olfactory organ and the 

 visceral clefts is quite as marked in the early stages of the 

 dogfish as in the chick; but to convey anything like an 

 adequate idea of it would require a much more extensive 

 series of figures than I am able to give here. 



Fig. 16 is taken from a longitudinal and vertical section 

 through the head of a dogfish embryo at stage m. The 

 section which is taken very near to the surface passes through 

 the auditory vesicle (aud.), parts of the trigeminal, facial, 

 auditory, glosso-pharyngeal and vagus nerves, the second 

 head cavity {h. 2), the eye (o.c), the mandibular, hyoid, and 

 first four branchial arches, as well as through the olfactory 

 pit {olf.). The section is a little deceptive, inasmuch as, owing 

 to the head being somewhat constricted just behind the eyes, 

 the buccal and lachrymal clefts do not appear at all, while 

 the constriction just referred to presents somewhat the ap- 

 pearance of a visceral cleft between the olfactory organ and 

 the mandibular arch, and might possibly be mistaken for 

 one on a superficial examination. The figure illustrates 

 well the resemblance between the olfactory organ, which is 

 larger than at a corresponding stage in the chick, and the 

 visceral clefts. 



Fig. 18 shows the same parts in a dogfish embryo of stage 

 o; as in the preceding figure, and for the same cause, the 

 buccal and lachrymal clefts do not appear in the section, but 

 the figure shows the general resemblance in position and 

 relations that exists between the olfactory organ and even the 

 hinder visceral, or branchial, clefts. 



