The development of the human body; a manual of human embryology . quent stages theembryonic ectoderm and somatic mesoderm at the edgesof the umbilicus grow out ventrally, carrying with themthe line of attachment of the amnion and forming a tubewhich encloses the proximal part of the yolk-stalk. Theectoderm of the belly-stalk at the same time extendingmore laterally, the condition represented in Fig. 63, C, isproduced, and, these processes continuing, the entire belly-stalk, together with the yolk-stalk, becomes enclosedwithin a cylindrical cord extending from the ventral sur-face of the body t
The development of the human body; a manual of human embryology . quent stages theembryonic ectoderm and somatic mesoderm at the edgesof the umbilicus grow out ventrally, carrying with themthe line of attachment of the amnion and forming a tubewhich encloses the proximal part of the yolk-stalk. Theectoderm of the belly-stalk at the same time extendingmore laterally, the condition represented in Fig. 63, C, isproduced, and, these processes continuing, the entire belly-stalk, together with the yolk-stalk, becomes enclosedwithin a cylindrical cord extending from the ventral sur-face of the body to the chorion and forming the umbilicalcord (Fig. 63, D). From this mode of development it is evident that thecord is, strictly speaking, a portion of the embryo, its sur-faces being completely covered by embryonic ectoderm,the amnion being carried during its formation further andfurther from the umbilicus until finally it is attachedaround the distal extremity of the cord. I40 THE DEVELOPMENT OF THE HUMAN BODY. d :m0 al V / V -iC *-y—7 v;.:. ^nJ| -uv -ua -yr. UV Fig. 65.—Transverse Sections of the Umbilical Cord of Embryosof (a) cm. and (b) 25 cm. al, Allantois; c, coelom; ua, umbilical artery; uv, umbilical vein; ys yolk-stalk. THE UMBILICAL CORD. 141 In enclosing the yolk-stalk the umbilical cord enclosesalso a small portion of what was originally the extra-embryonic body-cavity surrounding the yolk-stalk. Asection of the cord in an early stage of its development(Fig. 65, A) will show a thick mass of mesoderm occupyingits dorsal region; this represents the mesoderm of thebelly-stalk and contains the allantois and the umbilicalarteries and vein (the two veins originally present in thebelly-stalk having fused), while toward the ventral sur-face there will be seen a distinct cavity in which lies theyolk-stalk with its accompanying blood-vessels. Theportion of this ccelom nearest the body of the embryo be-comes much enlarged, and during the second month ofdevelopment con
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
