. The development of the chick; an introduction to embryology. Birds -- Embryology. 154 THE DEVELOPMENT OF THE CHICK conspicuous thickening, the optic chiasma, which is continued as a ridge in the lateral ventral zones on each side (Fig. 86). The infundibulum follows just behind this, and constitutes a considerable pouch-shaped depression from which the saccus infundibuli grows out later. The posterior wall of this depression rises sharply and joins the thickened tuberculum posterius which is the end of the floor of the diencephalon. The diencephalon is compressed laterally (Fig. 97); the dors
. The development of the chick; an introduction to embryology. Birds -- Embryology. 154 THE DEVELOPMENT OF THE CHICK conspicuous thickening, the optic chiasma, which is continued as a ridge in the lateral ventral zones on each side (Fig. 86). The infundibulum follows just behind this, and constitutes a considerable pouch-shaped depression from which the saccus infundibuli grows out later. The posterior wall of this depression rises sharply and joins the thickened tuberculum posterius which is the end of the floor of the diencephalon. The diencephalon is compressed laterally (Fig. 97); the dorsal zones are slightly thickened, indicating the future thalami Fig. 88. — Optical longitudinal section of the head of an embryo of 39 s. Abbreviations as before. The hypophysis should be mentioned here, although it is not embryologically a part of the brain. It arises as a median tubu- lar invagination of the ectoderm of the ventral surface of the head immediately in front of the oral plate at about the 20 s stage (Fig. 85), and grows rapidly inward in contact with the floor of the diencephalon. At about the 30 s stage its end reaches nearly to the infundibulum (Fig. 87). At first part of its wall is formed by the oral plate, and when this ruptures the effect is to shorten the apparent length of the hypophysis (Fig. 88). At about the 36 s stage its distal portion flattens laterally. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Lillie, Frank Rattray, 1870-1947. New York, Holt
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