32O FORMATION OF THE JAWS. 



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 pro- 

 bability 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 cho- 

 roid gland. The similarity of this organ to the pseudo-branch of the mandi- 

 bular or hyoid arch was pointed out to me by Dohrn, and the suggestion 

 was made by him that it is the remnant of a praemandibular 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 mouth ? I believe not. It is easy 

 to conceive that there may originally have been a praemandibular cleft behind 

 the suctorial mouth, but that this cleft gradually atrophied (for the same 

 reasons that the mandibular cleft shews a tendency to atrophy in existing 

 fishes, &c.), the rudiment of the gill (choroid gland) alone remaining to mark 

 its situation. After the disappearance of this cleft the suctorial mouth may 

 have become relatively shifted backwards. In the meantime the branchial 

 bars became developed, and as the mouth was changed into a biting one, the 



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 mandibular pseudo-branch to the choroid. For the relations 

 and structure of the choroid gland vide F. Miiller, Vergl. Anal. Myxinoiden, 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 pnemandibular segment 

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

 (mandibular segment). 



