. A text-book of agricultural zoology. Zoology, Economic. 416 EMBRYOLOGY-OP THE CHICK. quite closed. In front the neural tube has become swollen, forming the first cerebral vesicle, and from each side of this vesicle, by the rapid proliferation of the epiblastic cells, two processes grow out—the cyptic vesicles. A second and third vesicle then appear behind the first, and behind the third vesicle two pits, the auditory pits, the future organs of hear- ing. During the first half of the second day the heart (Ht) appears in the head-fold, its origin being connected with that cleavage of the mesob
. A text-book of agricultural zoology. Zoology, Economic. 416 EMBRYOLOGY-OP THE CHICK. quite closed. In front the neural tube has become swollen, forming the first cerebral vesicle, and from each side of this vesicle, by the rapid proliferation of the epiblastic cells, two processes grow out—the cyptic vesicles. A second and third vesicle then appear behind the first, and behind the third vesicle two pits, the auditory pits, the future organs of hear- ing. During the first half of the second day the heart (Ht) appears in the head-fold, its origin being connected with that cleavage of the mesoblast already referred to. It is formed in the pleuro-peritoneal space shown in fig. 200, and situated just beneath the developing fore-gut {al). At first the heart is flask-shaped,. -Twes. Fig. 200.—TKAiTSVERSE Section through Posterior Part of the Head of AN Embryo Chiok of Thirty Hours. if6, Hind-brain; Or, vagus nerve; al, throat; Hi, heart; Hy, hypoblast. Ihj, epiblast; not, notochord; tries, mesoblast; (After Foster and Balfour.) with a swelling at its anterior end and another behind, the future auricles. The heart is formed entirely from mesoblast, on the ventral side of the throat, by a curious process of hollowing out of the splanchnic mesoblast: some of the original central cells become blood - corpuscles. Concomitant with the appearance of the heart is the vascular system, which consists of a number of tubes hollowed out in the mesoblast. Blood-corpuscles are rapidly formed in masses in the vascular area, called blood-islands. By degrees the heart becomes S-shaped, the right-hand curve of. 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 Theobald, Frederick Vincent, 1868-1930. Edinburgh and London, W. Blackwood and sons
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1899
