The development of the human body; a manual of human embryology . tral 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 MESOD
The development of the human body; a manual of human embryology . tral 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 of the ventral mesoderm arrangethemselves in regular layers so as to enclose cavities whichappear in these regions (Fig. 58). The cavities of thesomites first formed become continuous with the cavitiescontained between the layers of the adjacent ventralmesoderm, but this continuity eventually disappears and. Fig. 59.—Transverse Section of an Embryo of mm. (See Fig. 42)showing on either side of the medullary canal a mesodermicSomite, the Intermediate Cell-mass, and the Ventral Meso-derm.—(von Lenhossek.) is not developed in the later formed somites. Each origi-nal lateral plate of gastral mesoderm then becomes dividedlongitudinally into three areas, a more median area com-posed of mesodermic somites, lateral to this a narrowarea underlying the original longitudinal groove whichseparated the somite area from the ventral mesoderm andwhich from its position is termed the intermediate cell 120 THE DEVELOPMENT OF THE HUMAN BODY. mass (Fig. 58, i), and, finally, the ventral last portion is now divided into two layers, the dorsalof which is termed the somatic mesoderm, while the ventralone is known as the splanchnic mesoderm (Fig. 58, so andsp; and Fig. 59), the cavity which separates these twolayers being the
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
