EVOLUTION OF ORGANS IN THE CHORDATA. 267 



such a pair) which in the actual development of Teleosteaus 

 never acquire an opening to the exterior. In a postscript to 

 this paper Dohrn mentions Hatschek's results concerning the 

 origin of the ciliated pit in Amphioxus. This pit is the left of 

 a pair of anterior evaginations of the endoderm, which opens 

 to the exterior while the other remains closed. According to 

 Dohrn these two diverticula are homologous with the hypo- 

 physis in the Teleostean, and the opening in Amphioxus is 

 the persistent branchial opening. The ciliated pit of the 

 Ascidians is also homologous with that of Amphioxus. Ac- 

 cording to Bateson the proboscis cavity with its pore in Balano- 

 glossus is homologous with the ciliated pit in Amphioxus, 

 but whether the body cavity of the proboscis in Balanoglossus 

 can be derived from a pair of gill-clefts is a question which 

 seems to threaten to do away with the possibility of the diagno- 

 sis of organs according to their embryological origin. 



The hypophysis in Petromyzon has a unique history in the 

 individual, and this forms the subject of the third member of 

 the series. The examination of the embryos of Petromyzon 

 was undertaken by Dohrn in order to prove that the funda- 

 mental difference generally supposed to exist between the 

 branchial cartilages of Selachians and of Petromyzon was 

 entirely imaginary, but the discussion of this subject is post- 

 poned till the hypophysis has been considered. Scott^ had 

 stated that the hypophysis of the Lamprey arose as an ecto- 

 dermal invagination connected with the nasal pit. 



Balfour had doubted this result, but Dohrn entirely confirmed 

 it, except that he found the hypophysial invagination to be at first 

 separate, lying between the commencing mouth and nasal cavity, 

 and that he pointed out that the whole long nasal duct of the 

 adult which runs back beneath the brain is as much part of the 

 hypophysis as the follicular organ formed from its inner ex- 

 tremity.2 The nasal duct is in fact a fused pair of ectodermal 



' ' Morph, Jahrb.,' vii. 



^ It seems extremely probable, although I am not aware that it has been 

 suggested before, that the nasal duct which in Myxine opens into the pharynx, 

 is homologous with the so-called nasal duct of Petromyzon. If this be so, of 



VuL. XXVII, PART 2. NEW SKR. X 



