April in,, 1879] 



NATURE 



593 



ON THE DEVELOPMENT OF THE SKULL 

 AND ITS NERVES IN THE GREEN TURTLE 

 ICHELONE MIDAS), WITH REMARKS ON 

 THE SEGMENT A TI ON SEEN IN THE SKULL 

 OF VARIOUS TYPES ^ 



FOR these embryos the author is indebted to Sir Wyville 

 Thomson and Mr. H. N. Moseley, the latter having sent 

 him the smaller specimens, and the former the ripe and nearly 

 ripe young. There are in all five stages. 



\st Stage, \-in, long. — The embryo is already fairly formed, for 

 there are rudiments of all the principal organs. About fifty-two 

 somatomes may be counted behind the head, and there are 

 evidently seven clefts — four post-oral, two pre-oral, and one oral. 

 The body of the embryo is tolerably distinct from the yolk-sac, 

 the mesocephalic flexure is well marked, and the tail is coiled 

 upon itself. 



The regions of the body, y\z., cervical, dorsal, and caudal, are 

 plain. A fold lying between the fore and hind limbs shows the 

 commencing carapace, which is at present the only mark to 

 distinguish it from any other Sauropsidan embrj'o. 



Half the ventral region is now taken up by the heart and its 

 pericardium, and behind it the ventral laminge have not closed 

 below, thus showing the Wolffian bodies within. Posteriorly the 

 umbilical vessels are seen emerging 



The limb-buds grow out from a continuous ridge, due to a 

 thickening of the me^oblast at the upper part of the somato- 

 pleure. 



The body-cavity extends into the head, thus corroborating 



Fig. I. — Chelotu midas, ist stage. Au, auditory capsule ; br. i and 2, 

 branchial arches; c, carapace; e, eye; _f.b, fore brain ;_f.l, fore-limb; 

 H, heart ; h.b, hind brain ; k.l, hind Umb ; hy, hyoir* • ni.b, mid brain ; 

 7Hn, mandible ; m.x.p, maxUlo-palatiae ; n, nostril ; u, uiibihcus. 



Mr. Balfour's account of the same thing taking place in the 

 Selachians. - 



As far as possible, the head has formed a coil similar to that 

 of the tail, from which it never more than partially recovers. 

 This bending of the head, which imprisons the elements of the 

 face, sets the dorsal region free, and the neural canal expands to 

 form the three vesicles of the brain. Buds are already seen on 

 the forebrain which give rise to the hemispheres. 



The axial tissues, below the nerv'ous structures, are thickening 

 into embryonic cartilage. 



A sectional view shows the mid-brain to be bent like a horse- 

 shoe, forming the "middle trabecula" of Rathke, which is 

 occupied by the apex of the notochord, its investing structures, 

 and the third nerves. 



Three pairs of chambers, for the organs of special sense, are 

 built in the sides of the cranium. 



The mouth is formed as in other types, by the extension of the 

 third pair of clefts into one another ; the fomrth or mandibulo- 

 hyoid cleft, being what is usually known as the first cleft. 



The first post-oral A-isceral arch, forms most of the frame- 

 work and machinery of the month, but as a rule, rudiments of 

 pre-oral arches, supplemented by sub-cutaneous bones, finish the 

 upper jaw. 



On the whole, the series of clefts and folds along the face of 

 the embryo are at this stage very regular, and the sense capsules 

 are intimately connected with those which lie below them, 



'Abstract of a paper by Prof. Parker. F.R.S., read at the Royal Society 

 on rebrunry 13. •' 



» This extension of the body-cavity is also seen m the lizard. 



The slit -like opening of the nasal sac and the space between 

 the eyeball and maxillo- palatine fold are very probably openings 

 of the same nature as those behind them. 



The tubular cartilage that forms round the external nostril is 

 homologous with the " labial " that serves the same purpose in 

 the Icthyopsida. 



The floor of the skull is open under the fore-brain, and the 

 double maxillo-palatine fold is sharply severed from the nasal 

 fold. The fronto-nasal process is but little freed from the in- 

 ferior cranial wall and the nasal folds. 





Fig. i.—Chel(me midas, 2nd stage. Letters as before. 



2nd Stage, ^-in. long. — The proximal part of each limb now 

 lies adherent to the infero-lateral region of the body in a manner 

 very similar to what is seen in the osseous fishes. The edge 

 above the deep sulcus between the marginal row of cutaneous 

 folds and the ingrowing abdominal part of the body-wall is 

 ultimately somewhat bevelled down, but it shows well that the 

 structure in which the plastron is formed is not originally flat but 

 trough-shaped. 



The upper region of the mandible is already assuming the 

 very image of the quadrate with its tympanic cavity and its 



Fig. 3. — CMone midas, 3rd stage. Letters as before, r, rostrum. 



condyles. In the cavity a discoid body — the " extra- stapedial " has 

 developed. Between the second post-oral (hyoid) fold and the 

 hollow of the quadrate elevation, the beginning of the membrana 

 tympani is seen. 



The eyeball is now at its relatively largest size, exceeding the 

 mid-brain in bulk. 



The maxillo-palatine fold is somewhat hoiu'-glass shaped, and 

 has a head -cavity in its fore part. 



In the middle of the palate an open space appears into which 

 the oral lining has grown. This diverticulum is the rudiment of 

 the pituitary body. It lies where the hypoblast and epiblast 



