. The development of the chick; an introduction to embryology . on. M. B., Mitl-l)rain. op. Ves., Oi)ticvesicle, pr. str., Primitive streak, s 2, s 4,s 16, Second, fourth, and sixteenth o. m., omi)halo-mescntcric vein. VII-VIII,The acustico-facialis i)rini()nlium. IX-X,Primonhum of the glossopharyngeus andvajrus. Tlie entire trunk tends ulso to liend ventrally, to developa dorsal convexity, and this apjjroxiniates its jjosterior end to thetip of the head. These flexures are characteristic of amniote FROM TWELVE TO THIRTY-SIX SOMITES 135 vertebrate embryos; the cause appears to
. The development of the chick; an introduction to embryology . on. M. B., Mitl-l)rain. op. Ves., Oi)ticvesicle, pr. str., Primitive streak, s 2, s 4,s 16, Second, fourth, and sixteenth o. m., omi)halo-mescntcric vein. VII-VIII,The acustico-facialis i)rini()nlium. IX-X,Primonhum of the glossopharyngeus andvajrus. Tlie entire trunk tends ulso to liend ventrally, to developa dorsal convexity, and this apjjroxiniates its jjosterior end to thetip of the head. These flexures are characteristic of amniote FROM TWELVE TO THIRTY-SIX SOMITES 135 vertebrate embryos; the cause appears to lie in the precociousdevelopment of the central nervous system, of which more here-after. Only the cranial flexure remains as a permanent con-dition. II. Origin of the Embryonic Membranes The period from about 12 to 36 somites also includes the earlyhistory of the embryonic membranes, amnion, chorion, yolk-sacand allantois. The first three arise from the extra-embryonicblastoderm, and the allantois arises as an outgrowth of the ven-tral wall of the \ Fig. 72. — The head of the same embryo frombelow,a. i. p., Anterior intestinal portal. B. a.,Bulbus arteriosus. Inf., Infundibiilum. ,Oral plate. Tr. a., Truncus , Ventricle, v. Ao., Ventral aorta. Origin of the Amnion and Chorion. The amnion is a thinmembranous sac, forming a complete investment for the embryoand continuous with the body-wall at the umbilicus; it lies beneaththe chorion to which it remains attached throughout incubationby a plate of tissue (sero-amniotic connection), and it arises incommon with the chorion from the extra-embryonic entire somatopleure external to the embryo is used up inthe formation of these two membranes. The amnion arises froma portion immediately adjoining the embryo itself; the remainderof the somatopleure peripheral to the amniogenous part formsthe chorion. Thus the extra-embryonic somatopleure may bedivided into two zones; an amniogenous zo
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