. Journal of anatomy. close agiee-ment between the two embrj^os. The brain is flexed at two points, the hind-brain is closed, while the mid-and fore-brains are still widely open. The anterior wall of the fore-gut lies against the floor of the fore-brainand forms a well-marked angle with the roof. It is somewhat thickened,and is presumably derived from a protochordal plate as in Perameles. At the anterior end of the notochord is a thickened mass of cells whichis bent over so that its anterior extremity lies between the floor of the fore- 186 Dr Katharine M. Parker brain and the anterior wall of
. Journal of anatomy. close agiee-ment between the two embrj^os. The brain is flexed at two points, the hind-brain is closed, while the mid-and fore-brains are still widely open. The anterior wall of the fore-gut lies against the floor of the fore-brainand forms a well-marked angle with the roof. It is somewhat thickened,and is presumably derived from a protochordal plate as in Perameles. At the anterior end of the notochord is a thickened mass of cells whichis bent over so that its anterior extremity lies between the floor of the fore- 186 Dr Katharine M. Parker brain and the anterior wall of the fore-gut, while the apex of the bend liesabove the antero-dorsal angle of the gut. This mass of cells represents theprechordal plate of Perameles, but is less well-marked, and, although itsanterior extremity is in close contact with the wall of the fore-gut, theredoes not appear to be actual continuity between the two structures atthis stage. On each side of the embryo, lying posterior to the well-developed optic. Fig. 3.—Stage II. Bettoncjia gaimardi. Longitudinal section passing througli tlieoptic groove and premandibular somites. SI. 2-2-2. H., hypophysial angle: 31., mesenchyme ; , medullary plate ; , optic groove ;, oral plate ; , premandibular somite ; Vv., blood-vessels. groove (fig. 3, ), there is a mesodermal mass wliich can readily be dis-tinguished by its relative density from the loose mesenchyme of the head(fig. 3, ). Furthermore, each of these mesodermal masses containsseveral small but perfectly distinct cavities, two of which are shown infig. 3, round which the cells are arranged in a radiating manner, formingan irregular epithelial wall. These cavities are not symmetrical on thetwo sides of the embryo. Owing to the fact that the head of the embryois cut longitudinally, it is very difficult to determine the exact lateral andmedian limits of these structures, but after careful study of the series, I The Development of the Hypophys
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Keywords: ., bookcentury1800, bookdecade1860, booksubjectanatomy, bookyear1867
