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Show 66 ON CLASSIFICATION. the anterior region of the body, grow up into pla.tes-tho dorsal l am1·n re (F1' 0o· • 31 , B) '· and these . dorsal .l am1nr.o , at l'e1 ngth uniting, inclose the future cerebr?-sp1nal c~v1ty (F1g. 31, 0, ?)· The blastoderm, beyond the region at wluch the dorsal.laminre are developed, grows downwards to !orm the ventral lam1nro, and where the ma~·gins of these pass 1nt~ the ge:neral blastoder.m, the outer serous, or epidermic, layer rises up Into a fold, wluch encircles the whole embryo; and the anterior and posterior parts of this fold growing more rapidly than the lateral portions, form a kind of hood for the cephalic and caudal ends of the body respectively (Fig. 32, E). The margins of the hoods and of Fig. 32. Fig. 32.-Developmeut of the Chick. . E. Embryo at the third day; g, heart; h, eye ; i, ear; ll, visceral arche~ and cl~f\s' l, m anterior and posterior folds of the amnion, which have not yet umted uvel ~ 1~ body' · 1 2 3 first second and third cerebral vesicles ·, la, vesicle of the thtr ventri'c le'. ' ' ' ' F. Chick at the fifth day ; n, o, rudiments of the anterior and posterior extremities ; Am, amnion; All, allantois; Um, umbilical vesicle. G. Under view of the head of the embryo F, the first visceral arch being cut away. .. THE REPTILIA. 67 their lateral continuations at length meot over the middle line ~f the body, and there coalesce: so that the body is covered for a while by a double sac, the inner layer of which is fonned by that vvall of the fold of the serous layer which is inferior, or nearest to the body of the embryo; while the outer layer is formed by that wall which,. is superior, or furthest _from the body of the embryo. The outer layer eventually disappears as a distinct structure, while the inner remains as the amnion. From the mode of formation which has been described, it results that the amnion is a shut sac, enveloping the body of the embryo ; and is continuous, on the ventral side of the body, with the integument of a region which eventually becomes the umbilicus (Fig. 32, F). The allantois is developed much later than the amnion, neither from the serous nor from the mucous (or epidermic and epithelial) layers of the germ, but from that intennediate stratum whence the bones, museles, and vessels are evolved. It arises, as a solid masR, from the under part of the body of the embryo, behind the primitive intestinal cavity; and, enlarging, becomes a vesicle, which rapidly increases in size, envelopes the whole embryo, and, being abundantly supplied with arterial vessels from the aorta, serves as the great instrument of respiration during footal life; the porosity of the egg-shell allowing the allantoic blood to exchange its excess of carbonic acid for oxygen by osmosis. The amnion and the external part of the allantois are thrown off at bh'th. That which has just been stated respecting the development and characters of the amnion and allantois of the chick is true not only of all Birds) but of all Reptilia. All embryonic REPTILIA are provided with an amnion and an allantois, like those just described in the footal fowl. In the embryonic state, also, they possess visceral arches and clefts, but no respiratory tufts are ever developed in the arches, nor are reptiles endowed with an apparatus for breathing the air dissolved in water at any period of their existence. 1'he skull of · all Reptilia is articulated with the vertebral column by a single condyle, into which the ossified basi-occipital enters largely F 2 |