392 DEVELOPMENT OF ELASMOBRANCH FISHES. 



gradually to travel round, so as still to maintain their primitive 

 position, while in successive generations a rudimentary spinal 

 furrow carrying with it the retina became gradually converted 

 into a canal 1 . 



If Dr Dohrn's comparison of the vertebrate nervous system 

 with that of segmented Annelids be accepted, the following two 

 points must in my opinion be admitted : 



(1) That the formation of the cerebro-spinal canal was sub- 

 sequent to the loss of the old mouth. 



(2) That the position of the old mouth is still unknown. 

 The well-known view of looking at the pituitary and pineal 



growths as the remnants of the primitive oesophagus, has no 

 doubt some features to recommend it. Nearly conclusive against 

 it is the fact that the pituitary involution is not, as used to be 

 supposed, a growth towards the infundibulum of the hypoblast 

 of the oesophagus, but of the epiblast of the mouth. It is almost 

 inconceivable that an involution from the present mouth can 

 have assisted in forming part of the old oesophagus. 



There is a view not involving the difficulty of the cesophageal 

 ring, fresh mouth 2 , and of the change of the ventral to the dorsal 



1 Professor Huxley informs me that he has for many years entertained somewhat 

 similar views to those in the text about the position of the rods and cones, and has 

 been accustomed to teach them in his lectures. 



2 Professor Semper ("Die Verwandtschaftsbeziehungen d. gegliederten Thiere," 

 Arbeiten aus d. Zool.-zoot. Institut, Wiirzburg, 1876) has some interesting speculations 

 on the difficult question of the vertebrate mouth, which have unfortunately come to 

 my knowledge too late to be either fully discussed or incorporated in the text. These 

 speculations are founded on a comparison of the condition of the mouth in Turbel- 

 larians and Nemertines. He comes to the conclusion that there was a primitive 

 mouth on the cardiac side of the supra-resophageal ganglion, which is the existing 

 mouth of Turbellarians and Vertebrates and the opening of the proboscis of Nemer- 

 tines, but which has been replaced by a fresh mouth on the neural side in Annelids 

 and Nemertines. In Nemertines however the two mouths co-exist the vertebrate 

 mouth as the opening of the proboscis, and the Annelid mouth as the opening for the 

 alimentary tract. This ingenious hypothesis is supported by certain anatomical facts, 

 which do not appear to me of great weight, but for which the reader must refer 

 to the original paper. It no doubt avoids the difficulty of the present position of the 

 vertebrate mouth, but unfortunately at the same time substitutes an equal difficulty in 

 the origin of the Annelidan mouth. This Professor Semper attempts to get over by 

 an hypothesis which to my mind is not very satisfactory (p. 378), which, however, 

 and this Professor Semper does not appear to have noticed, could equally well be 

 employed to explain the origin of a Vertebrate mouth as a secondary formation subse- 

 quent to the Annelidan mouth. Under these circumstances this fresh hypothesis does 



