10 THE VOYAGE OF H.M.S. CHALLENGER. 



cranial bar (preuasal cartilage), which curves clown into this lobe. In many Vertebrata 

 this lobe contains both the paired and single elements of this fore part of the cranial 

 floor; in this type only the single rod (PI. II. fig. 5, p.n.,i-tr.), the other bars (" cornua 

 trabeculge "), stopping short of this region. 



Without further dissection than the removal of the lower part of the post-oral folds 

 several important things can be seen (PI. 11. fig. 3). 



The double oral cleft has been laid open at the angles ; behind and within these 

 the Eustachian openings of the first post-oral or tympanic cleft (eu.) are seen ; they are 

 wide apart, and crescentic, with the concavity on the outer side. They are in this 

 view evidently comparable with the pre-oral chinks. 



These latter spaces, both open on each side, over the fore part of the oral recess ; 

 between them the palate is carinate ("vomerine region"), and half-way behind the 

 frouto-nasal fold an open space appears, into which the oral lining has grown, as an 

 indrawn, tubular recess. 



This peculiar diverticulum is the rudiment of the pituitary body; it stands on 

 the debatable ground where the hypoblast and epiblast meet, but according to the best 

 observers is formed from the latter layer of the blastoderm (see Balfour's Elasmo- 

 branchs, p. 189). 



The upper view (Plate II. fig. l) shows the size and front position of the mid-brain 

 (C 2), the oblique position of the huge eyeballs (e), the superorbital folds {s. ob.), and the 

 three tracts covering the hind-brain (C 3), whose upper part is very thin, and thinly 

 covered with skin. 



Also we see the auditory sacs (cm.), the tympanic region [co.), and the occipital and 

 cervical muscle-plates. In the halved head, vertically cut (fig. 4), we see the effects 

 of the cranial flexure at its uttermost degree of development ; the sigmoid flexure 

 of the hind-brain, the looped form of the mid-brain, and the low position of the fore- 

 brain. 



In these hand-sections the razor separates the cortical from the medullary matter 

 of the brain, which thus forms a coat that might be mistaken for the membrano- 

 cranium. 



The floor of the hind-brain (C 3) is thick, but its roof is thin ; in front, over it, the 

 cerebellar folds (C 3a) are forming. 



Also the mid-brain is folded forwards and backwards, within, whilst at its middle 

 it stretches over a large vertical space in which ascend the posterior clinoid wall {p.cl.), 

 and the notochord (nc.) ; down it the third nerve descends to the orbital muscles ; 

 the interspaces are filled with a gelatinous stroma. The highest part of this cavity in 

 the folded mid-brain is acute, behind that it is rounded, where the swelling base of the 

 hind-brain retires as it ascends to the cerebellum. 



The fore-brain (C 1) has developed median vesicles, and three pairs of vesicles. 



