. The development of the chick; an introduction to embryology. Birds -- Embryology. 292 THE DEVELOPMENT OF THE CHICK. the apex of the utriculus; and the horizontal canal formed from the external pocket extends between the separated lower ends of the other two. We must now proceed to a more detailed examination. In point of time the anterior (sagittal) semicircular canal is the first to be formed (Fig. 171); the external (hori- zontal or lateral) canal comes next, and considerably later the posterior (frontal). Thus the anterior canal is at first the largest, the external next, and the posterio
. The development of the chick; an introduction to embryology. Birds -- Embryology. 292 THE DEVELOPMENT OF THE CHICK. the apex of the utriculus; and the horizontal canal formed from the external pocket extends between the separated lower ends of the other two. We must now proceed to a more detailed examination. In point of time the anterior (sagittal) semicircular canal is the first to be formed (Fig. 171); the external (hori- zontal or lateral) canal comes next, and considerably later the posterior (frontal). Thus the anterior canal is at first the largest, the external next, and the posterior the smallest. These differences are, however, largely compensated in the course of the embryonic development. The ampullae appear as dilations in the pockets even before the canals are Fig. 170. — Model of the auditory formed, and are conspicuous dila- labyrinth of a chick embryo of 6 tions by the time that the central days and 17 hours; external vie%Y. parts of the pockets have broken (After Rothig and Brugsch.) throuo-h (Fio-. 172). C. a., Pocket for formation of ^r" ^ ^^ -,.,01 i anterior semicircular canal. C. 1., I^igs. 1/0-173 show the pockets Pocket for formation of lateral ^nd canals at six davs seventeen semicircular canal. C. p., Pocket ^ . for formation of posterior semicir- hours, seven days seventeen hours, cular canal. D. c Ductus coch- giprht davs seventeen hours, and leans. D. e., Endolymphatic duct. , " , ^ La., Lagena. eleven days seventeen hours. It will be seen that, whereas the an- terior and lateral canals are formed from the start in planes at right angles to one another, viz., the sagittal and horizontal, the posterior canal is not at first in the third or frontal plane, but gradually assumes it. The form of the utriculus is gradually assumed during the formation of the semicircular canals; it becomes drawn out into a roughly triradiate form, so that it consists of a central cavity and three sinuses, viz., the median sinus which receives th
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