Quain's elements of anatomy . .—Transverse section through the embryo of the chick and blastoderm onTHE SECOND DAY. (From Kolliker.) dd, hypoblast ; cli, chorda dorsalis ; rcw, primordial vertebras; mr, medullary plates ;Tl, corneous layer or epiblast ; uwh, cavity of the j^rimordial vertebral mass ; mp, meso-blast dividing at sp into hpl, parietal, and df, visceral laminte ; img. Wolffian ductbeginning in the intermediate cell-mass. 3. ITotocliord.—The third rudiment of the embryo is the chordadorsalis of von Baer or the notochord of more recent authors, which liesin the axis of the vertebral
Quain's elements of anatomy . .—Transverse section through the embryo of the chick and blastoderm onTHE SECOND DAY. (From Kolliker.) dd, hypoblast ; cli, chorda dorsalis ; rcw, primordial vertebras; mr, medullary plates ;Tl, corneous layer or epiblast ; uwh, cavity of the j^rimordial vertebral mass ; mp, meso-blast dividing at sp into hpl, parietal, and df, visceral laminte ; img. Wolffian ductbeginning in the intermediate cell-mass. 3. ITotocliord.—The third rudiment of the embryo is the chordadorsalis of von Baer or the notochord of more recent authors, which liesin the axis of the vertebral column and cranial base. When somewhatadvanced, it extends through the forepart of the embryonic area from the NOTOCHOED. 791 anterior extremity of the primitiye streak to uhe front of the medullarycanal; but, except at its hinder end, it has no immediate connectionwith the epiblast in which that canal is formed, and lies beneath it inthe place which is afterwards occupied by the bodies of the yertebra Fig. 686. ^^. Fig. 686. DiAGRA:JIMATIC LONGITUDINAL SECTION THROUGH THE AXIS OP AN EMBRYO-CHICK. (From Foster and Balfour.) Tlie section is supposed to be made at a time when the head-fold has commenced, butthe tail-fold has not yet appeared. A, epiblast ; B, mesoblast ; C, hypoblast; FSo, foldof the somatopleure ; FSp, fold of the splanchnopleure ; Am, commencing (head) fold ofthe amnion : NO, neural canal, closed in front, but still open behind ; Ch, notochord,—in front of it, uncleft mesoblast in the base of the cranium; D, the commencing foregut,or alimentary canal; Ht, heart ; i^i), pleuro-peritoneal cavity. and base of the cranium. When fully formed it is also separate fromthe hypoblast which runs across below it. From this it might appear atfirst sight that it belongs to the mesoblast, by a part of which it issomewhat later surrounded. But there are still doubts with respectto its origin, that is, whether it is from the mesoblast or hypoblast. Theformer view i
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