Embryo Chick at Fourth Day

Embryo chick at fourth day, viewed as a transparent object, lying on its left side. CH, cerebral hemispheres; FB, forebrain or vesicle of third ventricle, with Pn, pineal gland projecting from its summit; MB, midbrain; Cb, cerebellum; IV V, fourth ventricle; L, lens; chs, choroidal slit; Cen V, auditory vesicle; s m, superior maxillary process; 1F, 2F, 3F,4F., first, second, third, and fourth visceral folds; V, fifth nerve, sending one branch (ophthalmic) to the eye, and another to the first pharyngeal nerve, passing to the third visceral arch; G.Pg, pneumogastric nerve, passing towards the fourth visceral arch; iv, investing mass; ch, notochord; its front end cannot be seen in the living embryo, and it does not end as shown in the figure, but takes a sudden bend downwards, and then terminated in a point; Ht, heart seen through the walls of the chest; MP, muscle-plates; W, wing, showing commencing differentiation of segments, corresponding to arm, forearm, and hand; H L, hind-limb, as yet a shapeless bud, showing no differentiation. Beneath it is seen the curved tail.

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