170 HOMOLOGIES OF THE VERTEBRATE NERVOUS SYSTEM. 



furrow carrying with it the retina became gradually converted 

 into a canaP. 



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 oeso- 

 phageal ring, fresh mouth ^, and of the change of the ventral to 

 the dorsal surface, which, though so far unsupported by any 



1 Professor Huxley informs me that lie has for many years entertained some- 

 what similar views to those in the text ahout the position of the rods and 

 cones, and has been accustomed to teach them in his lectures. 



2 Professor Semper i(Z)te Verwandtschaftsheziehungen d. gegliederten Thiere, 

 Arbeiten aus d. Zool.-zoot. Jnstitut,Wuvzhm-g, 1876) has some interesting specu- 

 lations on the difficult question of the vertebrate mouth, which have unfortu- 

 nately 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 Tm-bellarians and Nemertines, He comes to the conclusion 

 that there was a primitive mouth on the cardiac side of the supra-oesophageal 

 ganglion, which is the existing mouth of Turbellarians and Vertebrates and the 

 opening of the proboscis of Nemertines, but which has been replaced by a 

 fresh mouth on the neural side in Annelids and Nem-ertines. 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 diffi- 

 culty in the origin of the Annelidan mouth. This Professor Semper attempts to 

 get over by an hypothesis wliicli to my mind is not very satisfactory (p. 878), 

 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 subsequent to the Annelidan mouth. Under these circum- 

 stances this fresh hypothesis does not bring us very much nearer to a solution 

 of the vertebrate-annelid mouth question, but merely substitutes one difficulty 

 for another; and does not appear to me so satisfactory as the hypothesis sug- 

 gested in the text. 



At the same time Professor Semjoer's h}i3othesis suggests an explanation 

 of that curious organ the Nemertine proboscis. If the order of changes 



