. The development of the chick; an introduction to embryology . rst intersomitic groovedrawn from an entire mount in balsam bytransmitted light,a. c. v., Amnio-cardiac vesicle, a. o., In-ner margin of Area opaca. Ect., , Entoderm H. F., Head-fold. i. ,First intersomitic furrow, med. pi., Medullaryplate. Mes., Mesoderm, n. gr., Neural groove,pr. gr., Primitive groove. Pra, proamnion. in the next chapter. At the 12 s stage one such place of contactis already formed, lying a short distance in front of the thickenedectoderm destined to form the auditory pit. HEAD-FOLD TO TWELVE
. The development of the chick; an introduction to embryology . rst intersomitic groovedrawn from an entire mount in balsam bytransmitted light,a. c. v., Amnio-cardiac vesicle, a. o., In-ner margin of Area opaca. Ect., , Entoderm H. F., Head-fold. i. ,First intersomitic furrow, med. pi., Medullaryplate. Mes., Mesoderm, n. gr., Neural groove,pr. gr., Primitive groove. Pra, proamnion. in the next chapter. At the 12 s stage one such place of contactis already formed, lying a short distance in front of the thickenedectoderm destined to form the auditory pit. HEAD-FOLD TO TWELVE SOMITES 95 Another place of fusion between the fore-gut and the ecto-derm is the so-called oral plate (pharyngeal membrane), whichoccupies a mid-ventral position at the extreme anterior parietal cavities meet posterior to the oral plate (Figs. 67and 75). Transverse sections show the oral plate to be depressedbeneath the level of the ventral surface of the head at the stage of10 somites (Fig. 55), a condition that increases, as development Fig. 49. — Median sagittal section of the head at the stage of 4 s. a. i. p., Anterior intestinal portal. F. G., Fore-gut. Ect., , Entoderm. H. F., head-fold. Mes., Mesoblast. n. F., Neural fold,or. pi., Oral plate. proceeds, by the formation of the cranial fiexture, and by the up-growth of the tissues behind and at its sides; thus will be estab-lished a deep depression lined by ectoderm, the floor of which isformed by the oral plate, and which is destined to form a largepart of the mouth. The depression is known as the stomodseum. IIL Origin of the Neural TubeThe Medullary Plate. The medullary plate is the primordiumof the central nervous system. At the time of formation of thehead-fold it is broad in front and narrower posteriorly, endingopposite the posterior end of the primitive streak. Its centralportion is not a separate plate of cells in the region of the primi- 96 THE DEVELOPMENT OF THE CHICK tive s
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Keywords: ., bookauthorlilliefrankrattray187, bookcentury1900, bookdecade1910
