OX THE AXCESTltAL FORM OF THE CHORD AT A. 265 



of this woi-k ; but, while recognizing that there is much to be said in favour 

 of the interchange of the dorsal and ventral surfaces, I am still inclined 

 to hold that the difficulties involved in this view are so great that it must, 

 provisionally at least, be rejected ; and that there are therefore no reasons 

 against supposing the present vertebrate mouth to be the primitive mouth. 

 There is no embryological evidence in favour of the view adopted by 

 Dohrn that the present mouth was formed by the coalescence of two clefts. 



If it is once admitted that the present mouth is the primitive mouth, 

 and is more or less nearly in its original situation, very strong evidence 

 will be required to shew that any structures originally situated in front 

 of it are the remnants of visceral clefts ; and if it should be proved that 

 such remnants of visceral clefts were present, the views so far arrived at 

 in this section would, I think, have to be to a large extent reconsidered. 



The nasal pits have been supposed by Dohrii to be remnants of visceral 

 clefts, and this view has been maintained .in a very able manner by Marshall. 

 The arguments of Marshall do not, however, appear to me to have any 

 great weight unless it is previously granted that there is an antecedent 

 probability in favour of the presence of a pair of gill-clefts in the position of 

 the nasal pits; and even then the development of the nasal pits as epiblastic 

 involutions, instead of hypoblastic outgrowths, is a serious difficulty which 

 has not in my opinion been successfully met. A further argument of 

 Marshall from the supposed segmental nature of the olfactory nerve has 

 already been spoken of. 



While most of the structures supposed to be remains of gill- clefts in 

 front of the mouth do not appear to me to be of this nature, there is one 

 organ which stands in a more doubtful category. This organ is the so- 

 called choroid gland. The similarity of this organ to the pseudo-branch 

 of the mandibular or hyoid arch was pointed out to me by Dohrn, and 

 the suggestion was made by him that it is the remnant of a pra?man- 

 dibular gill which has been retained owing to its functional connection 

 with the eye 1 . Admitting this explanation to be true (which however is by 

 no means certain) are we necessarily compelled to hold that the choroid 

 gland is the remnant of a gill-cleft originally situated in front of the 

 month 1 I believe not. It is easy to conceive that there may originally 

 have been a prsemandibular cleft behind the suctorial mouth, but that this 



1 The probability of the choroid gland having the meaning attributed to it by 

 Dohrn is strengthened by the existence of a prsemandibular segment as evidenced by 

 the presence of a prsemandibular head-cavity, the walls of which as shewn by Marshall 

 and myself give rise to the majority of the eye-muscles and of a nerve (the third nerve, 

 cf. Marshall) corresponding to it ; so that these parts together with the choroid gland 

 may be rudiments belonging to the same segment. On the other hand the absence of 

 the choroid gland in Ganoidei and Elasmobranchii, where a mandibular pseudo-branch 

 is present, coupled with the absence of a mandibular pseudo-branch in Teleostei where 

 alone a choroid gland is present, renders the above view about the choroid gland 

 somewhat doubtful. A thorough investigation of the ontogeny of the choroid gland 

 might throw further light on this interesting question, but I think it not impossible 

 that the choroid gland may be nothing else but the modified mandibular pseudo-branch, 

 a view which fits in very well with the relations of the vessels of the Elasmobranch 

 maudibular pseudo-branch to the choroid. For the relations and structure of the 

 choroid gland vide F. M tiller, Vergl. Anat. Myxinoideii, Part in. p. 82. 



It is possible that the fourth nerve and the superior oblique muscle of the eye which 

 it supplies may be the last remaining remnants of a second prsemandibular segment 

 originally situated between the segment of the third nerve and that of the fifth nerve 

 (mandibular segment). 



