1879.] Skull and its Nerves in the Green Turtle. 



335 



the order of time in the appearance of these parts in the quasi-ancient 

 stages of the early embryos of existing, but low, vertebrate types. 



The ventral wall of the head undergoes dehiscence in three places, 

 -on each side, in front of the tympano- eustachian, or so-called first 

 cleft. 



The right and left clefts directly in front of that open freely into 

 each other below ; they form the angles of the opening mouth. 



Another cleft, the lacrymal, is formed in the region over which the 

 third nerve forks ; this nerve, the motor oculi, is a true segmental 

 nerve, but is specially devoted to the eyeball. 



The eyeball forms for itself a nest above and in front of the mouth, 

 and this cupped orbital space is permanently open antero-inferiorly. 



In the turtle, at my second stage, the maxillo-palatine fold is very 

 large, is dilated at both ends, and pinched in the middle ; in the hind 

 part there is for a time, as I have showed in the lizard, an extension 

 of the pleuro-peritoneal cavity. 



This is the only one of the head cavities which opens into its fellow 

 of the opposite side ; the presence of this cavity is as sure a sign of 

 segmentation as the forking of a segmental nerve over the space in 

 front of and above it. 



The thick front part of the visceral fold between the lacrymal and 

 nasal clefts does not acquire a cavity. 



The whole of this double fold over and in front of the mouth cleft is 

 largely aborted in the Selachians by a foregrowth of the mandibular 

 fold; bat in some sharks, as in Notidanus, and in all rays, a true 

 palatine cartilage is developed in that part of the face which is be- 

 tween the lacrymal and nasal clefts.* 



Between the lacrymal and oral clefts, and therefore between the 

 hinder fork of the third nerve and the true anterior fork of the fifth 

 nerve, a visceral cartilage appears in several types. 



In Scymnus, among the sharks, in Mtnopoma and Siredon, among 

 the Urodeles, in nearly all lizards, and in the Chelonians, a separate 

 cartilage appears in the hinder lobe of the maxillo-palatine fold, after 

 the disappearance of the head cavity. 



In the lamprey, and the larvse of the Batrachia, the extreme forward 



* This cartilage is distinct also in all the Urodeles ; and in the species of the genus 

 Bufo, after metamorphosis, and in all the rest of the Batrachians as a very definite 

 region of the sub-ocular arch. In the salmon, among the Teleostei, this cartilage is 

 separately developed at first, and this is evidently the rule in the Order. 



In the Siluroids {Doras, Clarias) this becomes a straight rod of bone, carrying 

 the tentacle -bearing, minute maxillary, in front, and lying over the pterygoid, and 

 mesopterygoid behind ; with these it never unites. 



In lizards, and many birds, there is a very definite palatine cartilage which appears 

 between the lacrymal and nasal clefts ; it remains cartdaginous in the former, but 

 in many birds (Musophaga, Dicholophus, Diomedea, &c.) acquires its own bony 

 centre. 



VOL. XXVIII. 2 B 



