EEPORT ON THE GREEN TURTLE. 17 



projects but little, as it is largely bent forward (see fig. 4, C 3), and the cerebellum 

 (C Set) is only rudimentary at present. Comparing this head with that of the last stage, 

 it is seen how rapidly the permanent form is being assumed, and how fast the various 

 folds of skin — eyelids, superorbital ridges, lips, &c. — are growing. The upper third of 

 the head is not at present protected even by cartilage (see section PI. III. fig. 5). 



(c.) Loioer view of head.— ^elow , we see that the maxillo-palatine folds have coalesced 

 with the fronto-nasal fold (compare PL III. fig. 3, with PL II. figs. 2, 3) to form the upper 

 cheek, in which afterwards the maxillary and jugal bones appear laterally, and the 

 premaxillaries in front. 



There is still a large cleft in front between the eyeball and these two conjoined 

 folds; this is part of the original "lachrymal cleft" {l.cl., see also PL II. figs. 2, 8) ; 

 the nasal cleft is near the weU-rimmed nasal aperture and passage {e.n). The eye, behind, 

 rests on a cushion of thickening tissue, which overlaps the ball almost to the iris ; this 

 is the large lachrymal gland (l-Q-)- Round the ball the upper and lower lids are now 

 clearly seen (figs. 1-3). 



A comparison of the head in this stage with that of the last will show how rapidly 

 the generalised " Sauropsidaii " head has been transformed into the specialised head of 

 a Chelonian. 



(d.) Vertical section of head. — A vertical section of the head, both with the brain in 

 situ, and also with it removed (PL III. figs. 4, 5), helps much to elucidate the nature of 

 the metamorphosis that has taken place in so short a time as between my fourth and 

 fifth stages. 



The cavities of the brain vesicles are decreasing rapidly ; the hemispheres (fig. 4, C la) 

 form the foremost part of the brain ; the hind-brain (C 2) looks upwards and backwards, 

 and is becoming very solid ; and the hind-brain has, above and in front, some convolu- 

 tions that form the rudiments of the cerebellum (C 3, 3a). ^ 



The folded form of the mid-brain gives rise to a large space which now looks upwards 

 and backwards, and which contains some very important parts soon to be described. 



The optic and olfactory nerves are solid, — the former was tubular, and the latter will 

 have a cavity, when the olfactory tract or vesicle is formed and has communicated with 

 the nerve. 



The infundibulum (figs. 4 and 4a, inf.) is forming, but like the pituitary body {py.) 

 is still imperfect ; moreover, these two bodies are at present quite separate from each 

 other.^ 



The chondrocranium (PL III. fig. 5) is now at its greatest perfection, and it has 



' In this section the cortical matter of the brain is shown as partly severed from the medullary part, which is 

 due to the tearing action of the razor. 



^ The structures here shown on the face of a solid section will be largely illustrated by figures taken from the 

 transparent sections made in three directions. 



(ZOOL. CHALL. E.XP. — PART V. — 1880.) E 3 



