268 PEOF. W. K. PAEKER ON THE STRUCTURE AJJD 



evidently a reptile, and nearly half its length is due to the growth of the coiled tail. 

 The number of somatomes has greatly increased, and the lobes of the face are now 

 confluent ; yet the mesocephalic flexure is still perfect, throwing the huge mid brain 

 (PL LXII. fig. 3, C") forwards, as though it were the end of the embryo. The 

 hemispheres (C") are very much elongated in front of the pineal region (jj/); the 

 hind brain (C'^) is still very long and thinly covered. 



The prenasal beak is now definitely formed, and projects considerably beyond the 

 brain, so that the outer nostrils (ol,e.») are carried away from the eyeballs; these 

 latter are immense, and are fitted with well-margined sockets. 



The auditory capsules (au) are still evident as ovoidal swellings on the outside, and 

 the involution (aq.v) is evident as a lipped slit ; a curtain hangs down over the closed 

 first cleft. 



The mouth (figs. 3, 4, 711) is now well formed and has large angles ; the mandibles 

 (mn) are still short as compared with the upper or maxillo-palatine margin of the 

 mouth ; the hyoid (hi/) lies some distance behind the mandibles, and the clefts are 

 nearly closed. When the inferior arches are removed, and the head is viewed from 

 below (PI. LXIII. fig. 3), we see how much the palatine region has developed in front 

 of the pituitary involution' (py). 



At the mid line, where the faucial and palatine territories meet, there is a sagitti- 

 form recess, in the centre of which the pituitary rudiment is to be found. 



The hinder tract is short, and passes right and left into the clefts, the first of which 

 (cl^) only is figured ; it lies within and behind the mandible (mn). The epiblastic 

 mucous membrane in front of this pair of clefts is now greatly elongated, narrows 

 towards the front, and then widens again somewhat. In each angle, in front, the 

 lacrymal "involutions" (l.t) are seen. Eight and left, the club-shaped maxillo-palatine 

 folds [mx.p) are growing inwards, especially in front, and these inner lobes contain 

 the rudiments of the hard palate. The upper palatine skin shows a median seam when 

 the two sides have grown together. The nasal sacs and their openings {ol, e.n) are still 

 quite inferior in position, and the lobes of the fronto-nasal process (f.n.p) are growing 

 small ; they now form the anterior margin of the palate, where the promaxillary teeth 

 will be. 



When the palatal and faucial skin has been removed, then the hardening basis 

 cranii is exposed (PI. LXIII. fig. 4) ; in relation to the early chondi'ocranium of the 

 Vertebrata, generally, this is a very important dissection. 



At present the notochord (nc) is half as long as the whole basal tract ; it was two 



' In my earlier papers I have been in the habit of calling certain passages about and in front of the month, as 

 •well as the mouth itself, by the term " clefts." Now, and for the future, I shall only call those passages deftg 

 that lie behind the mouth, and in wliich the epiblast unites with the hypoblast. In the mouth, pituitiix)- 

 rudiment, lachrymal, and nasal passages, the whole tract, outside and in, is merely epiblast ; these will be 

 called "involutions." 



