. The development of the chick; an introduction to embryology. Birds -- Embryology. 92 THE DEVELOPMENT OF THE CHICK. a m >>7^ O c-^ o ;: li o o ffi ^ ^ ^ o c c = ^ s o o +i o 1:: "^ M CQ ':^ '^ -^ -â â â -^ (0 ms^ f=H growth of the medullary plate, which causes the latter to extend forward above the thinner and more pliable membrane in front. The entoderm is attached to the inner surface of the anterior end of the medullary plate (Fig. 47), and is apparently carried forw^ard with the latter to form the anterior portion of the fore-gut. The actual form of the fold depends upon the me
. The development of the chick; an introduction to embryology. Birds -- Embryology. 92 THE DEVELOPMENT OF THE CHICK. a m >>7^ O c-^ o ;: li o o ffi ^ ^ ^ o c c = ^ s o o +i o 1:: "^ M CQ ':^ '^ -^ -â â â -^ (0 ms^ f=H growth of the medullary plate, which causes the latter to extend forward above the thinner and more pliable membrane in front. The entoderm is attached to the inner surface of the anterior end of the medullary plate (Fig. 47), and is apparently carried forw^ard with the latter to form the anterior portion of the fore-gut. The actual form of the fold depends upon the mechani- cal properties of the membranes concerned, especially the unequal thickness of their parts produced by unequal grow^th. Although the head-fold thus ap- pears to be a single fold involving the two primary la3'ers, it is con- venient, for purposes of description, to consider it as two separate folds, ectodermal and entodermal. The deepening of these folds takes place at the same rate up to the time when four somites are formed (Fig. 49). At about this time the paired primordia of the parietal cavity (amnio-carcUac vesicles), which ap- pear in the mesoblast in the lateral extensions of the head-fold (Fig. 50), push in towards the mid- dle line so as to separate the ecto- dermal and entodermal limbs (Figs. 52 and 58). When six somites are formed, these cavities fuse in the middle line, thus effecting a complete separation of the two limbs. The further progression of the head-fold, after this union, takes place separately in the two limbs. \. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Lillie, Frank Rattray, 1870-1947. New York, Holt
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