Quain's elements of anatomy . L 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.
Quain's elements of anatomy . L 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 is held by Kolliker, and for the higher vertebrates is in Fig. Fig. 687.—Transverse section through the embryonic region of the blastoderm OF A chick at the TIME OF THE FORMATION OF THE NOTOCHORD. (From Balfour.) ep, epiblast; hy, hypoblast; ch, notochord ; me, mesoblast ; n, nuclei of the germi-nal wall yk. some respects tenable : but in elasmobranchs Balfour has found thenotochord to arise very distinctly from the lower layer by an actualinfolding of a linear strip of the cells of that layer, and he, as well assome others, are disposed to attribute the origin of the notochordin the amniota to a similar source. With respect to this view, 792 FEAMEWOEK OF THE BODY. sd. it is to be observed that the hinder end of the notochord is directlycontinuous mth the front part of the primitive streak where themesoblast and epiblast are united, and if it does spring fi-om the lowerlayer in front of that, it is still possible that in that situation the meso-blastic elements have not yet been differentiated ftom the hypob
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