. The elements of embryology. Chickens -- Embryos. 118 THE THIRD DAY. [CHAP. 37, JSf). Like the lens and the labyrinth of the ear, they are Fig. Head of an Embryo Chick of the Third Day viewed sideways as an Opaque Object. (Chromic acid preparation.) Cerebral hemispheres. Vesicle of third ventricle. Mid-brain. Gb. Cerebellum. Medulla oblongata. N. Nasal pit. ot. otic vesicle in the stage of a pit with the opening not yet closed up. op. Optic vesicle, with I. lens and choroidal fissure. The small dot in the centre of the lens indicates the remnant of its external
. The elements of embryology. Chickens -- Embryos. 118 THE THIRD DAY. [CHAP. 37, JSf). Like the lens and the labyrinth of the ear, they are Fig. Head of an Embryo Chick of the Third Day viewed sideways as an Opaque Object. (Chromic acid preparation.) Cerebral hemispheres. Vesicle of third ventricle. Mid-brain. Gb. Cerebellum. Medulla oblongata. N. Nasal pit. ot. otic vesicle in the stage of a pit with the opening not yet closed up. op. Optic vesicle, with I. lens and choroidal fissure. The small dot in the centre of the lens indicates the remnant of its external opening. The superficial epiblast moulds itself to the form of the optic vesicle and the lens; hence the choroidal fissure, though formed entirely underneath the superficial epiblast, is distinctly visible from the outside. 1 F. The first visceral fold ; above it is seen a slight indication of the superior maxillary process. 2, 3, 4 F. Second, third and fourth visceral folds, with the visceral clefts between them. SO. Portion of the somatopleure springing from between the ends of the visceral folds. formed from the external epiblast; unlike them they are never closed up. At first they have no distinct connection with their respective olfactory vesicles, and their openings are independent and separate, there being as yet no actual mouth to connect them with each other. 14. It must be borne in mind that especially in the early stages of development, owing to the very unequal growth of different parts, the relative position of the various structures is continually shifting. This is very well seen in the instance of the heart. At its first appearance, the heart is lodged immediately beneath the extreme front of the ali- mentary canal so far forwards as to underlie that portion of the medullary canal which will form the brain. It is, in fact, at that epoch a part of the head. From that early position it gradually recedes farther and farther backward, until, at the end of the third d
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