MORPHOLOGY OF THE VERTEBRATE OLFACTORY ORGAN. 327 



In connection with this point, the study of whole embryos 

 affords evidence fully as striking as that yielded by sections. 

 I would here refer especially to certain of the figures given by 

 Professor Parker in his monograph on the " Structure and 

 Development of the Skull in Sharks and Skates," published 

 in the ' Transactions of the Zoological Society ' for 18T9 : 

 on Plate XXXIX side views of the heads of embryos of 

 Raia maculata are given (figs. 1 and 2), in which the re- 

 semblance between the slit-like aperture of the olfactory 

 organ and the gill slits is shown with remarkable distinct- 

 ness. The direction of the slit forms an angle of about 

 120° with the hyomandibular or spiracular cleft, which 

 angle is almost exactly that made by the longitudinal axis 

 of the forebrain with that of the hind brain, i.e. is the 

 amount of cranial flexure ; hence, but for cranial flexure, 

 the external slit-like aperture of the olfactory organ would 

 be parallel to the perf^-ctly similar gill slits. 



The figures of dogfish embryos of stages K and L, given 

 by Mr. Balfour on Plate VII of his monograph on Elasmo- 

 branch Fislies, illustrate the same points. The reference to 

 these two works acquires additional weight from the con- 

 sideration that the figures which I have named were drawn, 

 I have reason to believe, without the slightest intention on 

 the part of the authors to direct attention to the resem- 

 blance. Even in an adult skate the similarity between the 

 olfactory organ and the gill slits is sufficiently striking. 



The same points appear, if possible, still more clearly in 

 axolotl and salmon embryos, especially in the former. I 

 have found, however, that to give any adequate representa- 

 tion of these would require a large number of figures, 

 which figures would also serve to illustrate other points 

 in the development of the axolotl, which I hope to deal 

 with on some subsequent occasion. 



Fig. 30 represents a longitudinal and vertical section 

 through the head of a trout embryo on the thirtieth day 

 after fertilisation of the ova. The section passes through the 

 hind and midbrains and the eye (o. c). The ventral sur- 

 face of the section presents a series of undulatory folds, 

 corresponding to the bases of the visceral arches, with their 

 intervening clefts. The olfactory pit {olf.) is seen to form 

 the most anterior of these depressions, and to differ from the 

 hinder clefts in little but the greater thickness of its 

 epithelium, and the somewhat greater interval between it 

 and the next cleft. The cleft next but one to the olfactory 

 pit is that over which the trigeminal nerve forks — i. e. the 

 buccal or mouth cleft ; it is situated between the maxillary 



