Archive image from page 91 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 Mes. A Mi, Pig 36, _ a. Intermediate stage of the formation of the primitive streak of the sparrow. (After Sehauinsland.) B. Fully formed primitive streak of the spar- row. (After Sehauinsland.) a. o., Area opaca. a. p., Area pellucida. Ent. Th., Thickening of entoderm. Mes., Mesoderm, pr. f., Primitive fold. pr. gr., Primitive groove, pr. p., Primitive pit. pr. str., Primitive streak, s. gr., Sickle groove. At first the surface o

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Archive image from page 91 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 Mes. A Mi, Pig 36, _ a. Intermediate stage of the formation of the primitive streak of the sparrow. (After Sehauinsland.) B. Fully formed primitive streak of the spar- row. (After Sehauinsland.) a. o., Area opaca. a. p., Area pellucida. Ent. Th., Thickening of entoderm. Mes., Mesoderm, pr. f., Primitive fold. pr. gr., Primitive groove, pr. p., Primitive pit. pr. str., Primitive streak, s. gr., Sickle groove. At first the surface of the primitive streak is even, but, as it elongates, a groove appears down its center. This groove is known as the primitive groove; it is bounded by the primitive folds and terminates abruptly in front in a pit, the primitive pit, which corresponds to the neurenteric canal of other verte-

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