Archive image from page 114 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 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), which occupies a mid-ventral position at the extreme anterior end. The parietal cavities meet posterior to the oral plate (Figs. 67 and 75). Transverse sections show the oral plate to be depressed beneath the level of the ventral surface of the head at the stage of 10 somites (Fig. 55), a condi
Archive image from page 114 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 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), which occupies a mid-ventral position at the extreme anterior end. The parietal cavities meet posterior to the oral plate (Figs. 67 and 75). Transverse sections show the oral plate to be depressed beneath the level of the ventral surface of the head at the stage of 10 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., Ectoderm. Ent., Entoderm. H. F., head-fold. Mes., Mesoblast. n. F., Neural fold, or. pi., Oral plate. proceeds, by the formation of the cranial flexture, and by the up- growth of the tissues behind and at its sides; thus will ])e estab- lished a deep depression lined by ectoderm, the floor of which is formed by the oral plate, and which is destined to form a large part of the mouth. The depression is known as the stomodseum III. Origin of the Neural Tube The Medullary Plate. The medullary plate is the primordium of the central nervous system. At the time of formation of the head-fold it is broad in front and narrower posteriorly, ending opposite the posterior end of the primitive streak. Its central portion is not a separate plate of cells in the region of the primi-
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