735 



would not have neglected to point out, that, in the Holostei, we 

 have no reason to believe in the entodernial origin of the sense 

 organ, now tliat in the Dipnoi ') its formation from the et'toderni 

 is manifest. 



As opposed to Pinkus, Agar says "Tiiis organ has no relation to 

 the laleral line system of sense organs". To my opinion, however, 

 it undoubtedly belongs to this system, because it possesses a iieuro- 

 raast, is supplied by a brancii from the lateral line system of the 

 facial nerve, and moreover is clearly of ectodermal origin in the 

 Dipnoi. 



The majority of epidermal sense organs, sinks under the epidermis 

 during the ontogenetic period, and finds protection by the subcuta- 

 neous connective tissue. Only one organ having its seat of origin 

 in the immediate vicinity of the spiracle, sinks therein, acquiring a 

 considerable development. 



In my opinion this not only happens when the spiracle no longer 

 breaks through outwardly, retaining its opening into the gut, as in 

 the Holostei, but also, when it moreover loses its connection with 

 the gut, as in the Dipnoi. 



Ijet us now proceed to the Selachians. In these Wright examined 

 the spiracle of a 60 m.m. long embryo of Mustelus. Here he found 

 two diverticula, situated above each other, on the medial wall. The 

 dorsal diverticulum reached till under the canalis semicircularis late- 

 ralis of tiie auditory organ, and was already discovered in a number 

 of adult Selachians, by Joh. Müllek (1841). 



The ventral diverticulum did not reach the cranial cartilage, and 

 at one place contained columnar epithelium, which he took for 

 sense organ epithelium, and which according to him, was supplied 

 by the ram. praetrematicus of the facial nerve. This innervation 

 would lead us to expect, that we have here to deal with a different 

 sense organ to that in the Holostei. Phelps Aij.is, however, in 1901, 

 examined a 122 m.m. long embryo of Mustelus, and was able to 

 trace the nerve fz-om the organ till near the ram. oticus, the same 

 branch which also supplies the sense organ in the Holostei. 



Independent of Wright's work, that of van Bemmelen appeared 

 in the same year (1885). The latter, besides in Mustelus, found both 

 the diverticula in a great number of Selachians, in embryos as well 



') Greil (1913) mentions the ectodermal origin of the sense organ ("Hyoman- 

 dibular organ") in Geratodus, and its innervation by a branch from the lateral line 

 system ("ram. Ijypoticiis") of the facial nerve. Whether the sense organ in Geratodus 

 is afterwards also surrounded by the cranial cartilage, I do not find mentioned. 



