. The development of the chick; an introduction to embryology . loop, on the other hand, formsnumerous secondary convolutions (Fig. 180) which lie at first inthe umbilical cord, but which are gradually retracted (seven-teenth to eighteenth day) into the abdominal cavity. The two intestinal caeca begin to grow out as finger-shapedprocesses from the swelling already referred to, about the seventhday, and rapidly attain considerable length. The large intestineelongates only about in proportion to the growth of the entireembryo. Having thus noted the general gross anatomy of the embry- 312 THE DEV
. The development of the chick; an introduction to embryology . loop, on the other hand, formsnumerous secondary convolutions (Fig. 180) which lie at first inthe umbilical cord, but which are gradually retracted (seven-teenth to eighteenth day) into the abdominal cavity. The two intestinal caeca begin to grow out as finger-shapedprocesses from the swelling already referred to, about the seventhday, and rapidly attain considerable length. The large intestineelongates only about in proportion to the growth of the entireembryo. Having thus noted the general gross anatomy of the embry- 312 THE DEVELOPMENT OF THE CHICK onic intestine, we may next note a few details concerning someof its divisions. The history of the mesenteries is considered inChapter XI). (Esophagus. Owing to the rapid elongation of the neck theoesophagus quickly becomes a long tube. On the sixth day itslumen becomes very narrow, and on the seventh day completelyoccluded immediately behind the glottis, owing to proliferationof the lining cells. On the eighth day the occluded portion \. ■^ -y •-^?: Fig. 181. — Photograph of a transverse section through the oesopha-gus and trachea of an 8-day H., Copula of the hyoid. (Es., (Esophagus. Tr., jug., Jugular vein. extends only a short distance behind the glottis: it is com-pressed dorso-vent rally and extended laterally throughout theoccluded region (Fig. 181). On the eleventh day it is open againalong its entire length. The crop arises as a spindle-shaped dila-tation of the fx^sophagus at the base of the neck; on the eighthday it is about double the diameter of the parts immediately ALIMENTARY TRACT AND ITS APPENDAGES 313 in front of and behind it (Fig. 150). No detailed account of itsdevelopment exists. Stomach. It is well known that the stomach of birds exhibitstwo successive divisions, the pro vent riculus and the gizzard,the former of which has a digestive function and is richl}^ pro-vided with glands, while the latter has a
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