MORPHOLOGY OF THE VERTEBRATE OLFACTORY ORGAN. 329 



diameters, sliows also the spinal cord, with the anterior and 

 posterior roots of a spinal nerve, the notochord (n.), muscle 

 plates {m. p.), pharynx {al.), parts of the vagus nerve, the 

 cardiac and. dorsal aortse, and on either side the branchial 

 arteries {h. a.) The free surface of each branchial arch 

 presents a series of small equidistant folds, the rudiments of 

 the gills {g.), which, even under this low magnifying power, 

 have a close resemblance to the Schneiderian folds. 



In order to show this resemblance more satisfactorily I 

 have given figures of the parts in question on a larger scale. 

 Fig, 25 represents the right olfactory pit of fig. 23, and 

 fig. 26 the left gill of fig. 24; both figures are thus taken 

 from the same embryo, and the magnifying power employed 

 — ninety diameters — is the same in the two cases. 



These figures show that the correspondence between the 

 two structures is by no means confined to their coarser 

 anatomy, but extends even to histological details. The folds 

 are seen to be in the two cases — gills and Schneiderian folds 

 — of the same width, and the same distance apart; in both 

 cases, though consisting mainly of epithelium, they yet 

 involve the underlying mesoblast to a certain, though slight, 

 extent,^ but as nearly as possible to the same extent in the 

 two cases. The epithelium that forms the greater part of 

 the folds is of the same thickness in the two cases, and of 

 the same histological character, consisting mainly of columnar 

 cells in close contact Avith one another, and arranged, as a 

 rule, in two rows. 



The same folds are shown, at a somewhat later period, in 

 figs. 27 and 28, the former representing the Schneiderian 

 folds, the latter the gills of the same embryo. Though the 

 resemblances are still strong, there are now well-marked 

 differences between the two structures ; thus, in fig. 27 the 

 epithelium is somewhat thicker than in the gills, while the 

 mesoblast enters more largely into the gills than the Schnei- 

 derian folds. Most of the gill folds already present a central 

 blood-vessel ; it is very difficult- to satisfy oneself of the ex- 

 istence of distinct walls to these blood-vessels, which appear 

 in many cases to be simply channels in the mesoblast form- 

 ing the axis or core of each gill fold. Similar blood-vessels 

 exist, especially at a rather later stage, in the Schneiderian 

 folds, and their relations are similar to those in the gills. 



Even in adult Elasmobranchs the Schneiderian folds re- 

 semble the gills closely in their great vascular supply, in 



^ Balfour notes this in the case of the gills, but describes the Schnei- 

 derian folds as folds of epithelium. Op. cit., p. 184. 



