. Embryology. Embryology; Embryology; genealogy. DEVELOPMENT DURING THE FIRST DAY OF INCUBATION 209 Fig. 123. A blastoderm of about 16 hours removed from the yolk and stained. Various regions may be distinguished by their abilities to transmit light. The primitive streak lies along the long axis of a clear elliptical area. The clear area is the area pellucida. Bounding the area pellucida is the darker area opaca, in which the blood islands are de- veloping. At the anterior end of the primitive streak Hensen's node ap- pears as a somewhat denser spot. The streak appears lighter along its midlin
. Embryology. Embryology; Embryology; genealogy. DEVELOPMENT DURING THE FIRST DAY OF INCUBATION 209 Fig. 123. A blastoderm of about 16 hours removed from the yolk and stained. Various regions may be distinguished by their abilities to transmit light. The primitive streak lies along the long axis of a clear elliptical area. The clear area is the area pellucida. Bounding the area pellucida is the darker area opaca, in which the blood islands are de- veloping. At the anterior end of the primitive streak Hensen's node ap- pears as a somewhat denser spot. The streak appears lighter along its midline. A groove or depression— the primitive groove—runs along the midline. The future head struc- tures of the embryo are located in front of and around the anterior end of the streak. (Courtesy Mr. Lester A. Barter, Sloan-Kettering Institute.). the blood islands from whence come erythrocytes and some blood vessels. Although no definite, permanent structure is yet visible, many of the structures of the later embryo, such as heart and kidney, will self-differen- tiate if the proper cells are transplanted at this time. In this respect the late primitive-streak stage is similar to the late gastrula or early neurula of the amphibian. From the time of the primitive streak to 24 hours the first definitive structures become visible (Fig. 124, embryo about 3-3 mm. in length). The primitive nervous system, which first arose as a flat plate, has now folded to form the neural folds and neural groove. The neural folds have not come together in the anterior and posterior levels but do approximate each other in the middle regions. Under the head region the ectoderm and endoderm are folded posteriorly to give a pocket (Fig. 101). This fold is termed the head fold, and concomitantly with its formation the endoderm is folded back upon itself to form a pocket which is the cavity of the foregut. The opening into the foregut is the intestinal portal (Fig. 124). Figure 125 shows a cross. Please no
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Keywords: ., boo, bookcentury1900, booksubjectembryology, booksubjectgenealogy
