The development of the human body; a manual of human embryology . extending completely across theembryonic disk, but is divided into two lateral plates, inthe interval between which the ectoderm of the floor of themedullary groove and the chorda endoderm are in closecontact (Fig. 34). These lateral plates represent thegastral mesoderm, whose origin has already been described(p. 77), and which apparently supplants the originalprostomial mesoderm, whose fate in the human embryo is I iS THE DEVELOPMENT OF THE HUMAN BODY. at present unknown. The changes which now occur havenot as yet been observed
The development of the human body; a manual of human embryology . extending completely across theembryonic disk, but is divided into two lateral plates, inthe interval between which the ectoderm of the floor of themedullary groove and the chorda endoderm are in closecontact (Fig. 34). These lateral plates represent thegastral mesoderm, whose origin has already been described(p. 77), and which apparently supplants the originalprostomial mesoderm, whose fate in the human embryo is I iS THE DEVELOPMENT OF THE HUMAN BODY. at present unknown. The changes which now occur havenot as yet been observed in the human embryo, thoughthey probably resemble those described in other mamma-lian embryos, and the phenomena which occur in the sheepmay serve to illustrate their probable nature. The lateral plates increase in size by the multiplicationof the cells which compose them and, in sections, have asomewhat triangular form, the portions nearest the me-dian line of the embryo being much thicker than the morelateral parts. In the region which will later become the. Fig. 58.—Transverse Section through the Second Mesodermic Somite of a Sheep Embryo 3 mm. , Amnion; en, endoderm; i, intermediate cell-mass; mg, medullary groove; ms, mesodermic somite; so, somatic and sp, splanchnic layers of the ventral mesoderm.—{Bonnet.) neck of the embryo a longitudinal groove appears uponthe dorsal surface of each plate, marking off the moremedian thicker portion from the lateral parts, and themedian portions then become divided transversely into anumber of more or less cubical masses which are termedthe protovertebrce or, better, mesodermic somites (Fig. 58,ms), structures whose appearance in surface views hasalready been described (Figs. et seq.). The cells of thesomites and of the lateral parts, which may be termed the THE MESODERMIC SOMITES. II9 ventral mesoderm, are at first stellate in form, but laterbecome more spindle-shaped, and those near the center ofeach somite and those o
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
