The development of the human body; a manual of human embryology . inspection of Fig. 31than from many lines of description. If this process ofconcrescence really occurs, then the point where the origi-nal rudimentary blastopore occurred is now situated farforward upon the embryonic disk, and Hensens nodeindicates a proliferation of cells in the vicinity of theblastopore to form the prostomial mesoderm. 76 THE DEVELOPMENT OF THE HUMAN BODY. As regards the head process, it is a band of cells whichgrows forward from the region of the blastopore along themedian line and replaces the primary endode
The development of the human body; a manual of human embryology . inspection of Fig. 31than from many lines of description. If this process ofconcrescence really occurs, then the point where the origi-nal rudimentary blastopore occurred is now situated farforward upon the embryonic disk, and Hensens nodeindicates a proliferation of cells in the vicinity of theblastopore to form the prostomial mesoderm. 76 THE DEVELOPMENT OF THE HUMAN BODY. As regards the head process, it is a band of cells whichgrows forward from the region of the blastopore along themedian line and replaces the primary endoderm in thatsituation (Fig. 32, chp). It corresponds, therefore, tothe dorsal wall of the invagination of secondary endodermin the reptile, the ventral wall of the invagination notdeveloping at all, a condensation of development alreadyindicated in the turtle (see p. 69). Indeed, in the gecko,the turtle, and the mammal we have three degrees of sim-plification of a process. In the gecko a sac-like invagina-tion extends nearly to the anterior edge of the embryonic. § -V • ^•?—:i*w^ *••? «-*->?* m^ ? op. Fig. 32.—Transverse Section of the Embryonic Area of a Dogs Ovum at about the Stage of Development shown in Fig. 29, section passes through the head process (Chp) ; M, mesoderm.— (Bonnet.) disk and its ventral wall later disappears; in the turtlethe invagination is comparatively slight and the uselessventral wall is only partly developed; and, finally, in themammal (Fig. 33) the invagination is practically non-existent and no ventral wall whatsoever is formed, onlythe dorsal wall (ce) growing forward. It should be statedthat in some mammals apparently the most anterior por-tion of the roof of the archenteron is formed directly fromthe cells of the primary endoderm, which in this regionare not replaced by the head process, but aggregate to THE FORMATION OF THE GERM LAYERS. 77 form a compact plate of cells with which the anterior ex-tremity of the head process
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
