Scharff — On Dohrn's Theories on the Origin of Vertebrates. 15 



invertebrates. He located the ancestral mouth in the fourth 

 ventricle, and supposed that the crura cerebelli were the homologues 

 of the lateral commissures. However, in the first part of his 

 " Studies," published seven years after this pamphlet, Dohrn 

 acknowledges that he had committed a fault in enunciating such 

 an untenable supposition. Two investigators especially drew his 

 attention to it, viz. Professor Fritsch and Mr. Sanders. According 

 to Dohrn's original view, several cerebral nerves would have been 

 included in the supra-cesophageal ganglion. This induced the 

 author to abandon, for the present, the investigation as to the 

 position of the ancestral mouth, and to take up another problem, 

 namely, that of the development of the actual mouth. 



I. — The Origin of the Mouth. 1 



The results of researches made on embryos of bony fishes have 

 confirmed a view Dohrn arrived at, as far back as 1871. This 

 view was that the mouth of living vertebrates had originated from 

 a union of two gill-clefts. He observed in several teleostean 

 embryos that there was no stomodaeum or ectodermal oral in- 

 vagination. The mouth opened to the exterior, first on the sides, 

 the middle part being still closed. Hence there were two ready- 

 formed openings at first, which had an endodermal origin, before 

 the ultimate rupture of the part between had taken place. 



This concludes Dohrn's first Paper. 



II. — The Origin and Significance of the Hypophysis in 

 Teleosteans. 2 



Several of the more recent investigators on the development of 

 elasmobranch fishes, amphibia, and the higher vertebrates, agree 

 that the hypophysis is strictly ectodermal in its origin, being a 

 derivative of the stomodaeum. Dohrn having denied the existence 

 of a stomodaeum in teleosteans, it would evidently have been 

 difficult for him to derive the hypophysis from it. But actual 

 observation favoured his view in this case again. The hypophysis 

 makes its first appearance at the same time as the endodermal 



1 Ibid., vol. iii. 1882, pp. 252-263. 2 Ibid., vol. iii. pp. 264-279. 



