Archive image from page 154 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 FROM TWELVE TO THIRTY-SIX SOMITES 135 vertebrate embryos; the cause appears to lie in the precocious development of the central nervous system, of which more here- after. Only the cranial flexure remains as a permanent con- dition. 11. Origin of the Embryonic Membranes The period from about 12 to 36 somites also includes the early history of the embryonic membranes, amnion, chorion, yolk-sac and allantois. The first three arise from the
Archive image from page 154 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 FROM TWELVE TO THIRTY-SIX SOMITES 135 vertebrate embryos; the cause appears to lie in the precocious development of the central nervous system, of which more here- after. Only the cranial flexure remains as a permanent con- dition. 11. Origin of the Embryonic Membranes The period from about 12 to 36 somites also includes the early history of the embryonic membranes, amnion, chorion, yolk-sac and allantois. The first three arise from the extra-embryonic blastoderm, and the allantois arises as an outgrowth of the ven- tral wall of the hind-gut. m -Pr'am. CJy, vif-m v- Xl-P- Fig. 72. — The head of the same embryo from below. a. i. p., Anterior intestinal portal. B. a., Bulbus arteriosus. Inf., Infundibulum. , Oral plate. Tr. a., Truncus arteriosus. Ven., Ventricle, v. Ao., Ventral aorta. Origin of the Amnion and Chorion. The amnion is a thin membranous sac, forming a complete investment for the embryo and continuous with the body-wall at the umbilicus; it lies beneath the chorion to which it remains attached throughout incubation by a plate of tissue (sero-amniotic connection), and it arises in common with the chorion from the extra-embryonic somatopleure. The entire somatopleure external to the embryo is used up in the formation of these two membranes. The amnion arises from a portion immediately adjoining the embryo itself; the remainder of the somatopleure peripheral to the amniogenous part forms the chorion. Thus the extra-embryonic somatopleure may be divided into two zones; an amniogenous zone immediately adja-
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