The development of the human body; a manual of human embryology . 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 unites. S
The development of the human body; a manual of human embryology . 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 unites. Such a conditionwould represent a further modification of the originalcondition. As regards the formation of the mesoderm it is possibleto recognize both the prostomial and gastral mesodermin the mammalian ovum, though the two parts are not soclearly distinguishable as in lower forms. It has alreadybeen seen that Hensens node probably indicates the ex-istence of a mass of prostomial mesoderm, and when thehead process grows forward it carries with it some of this. Fig. 33.—Diagram of a Longitudinal Section through the Em-bryonic Disk of a , Amnion; ce, chorda endoderm; ec, ectoderm; nc, neurenteric canalps, primitive streak.—{Heape.) tissue. But, in addition to this, a contribution to themesoderm is also apparently furnished by the cells of thehead process in the form of lateral plates situated on eachside of the middle line. These plates are at first solid(Fig. 34, gm), but their cells quickly arrange themselves intwo layers, between which a ccelomic space later , as has already been pointed out, the layerof enveloping cells splits into two concentric layers, theinner of which seems to be mesodermal in its nature andforms a layer lining the interior of the trophoblast andlying between this and the primary endoderm. Thislayer is by no means so evident in the lower forms, but is 78 THE DEVELOPMENT OF THE HUMAN BODY. perhaps represented in the reptilia
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
