. The development of the chick; an introduction to embryology . ) /-■//■- Iu;. SS. — Optical loiiKitudiiial section of the head uf an embryo of 39 )breviati()ns as before. The hypophysis should be mentioned here, although it is notembryologically a part of the brain. It arises as a median tubu-lar invagination of the ectoderm of the ventral surface of thehead immediately in front of the oral plate at about the 20 sstage (Fig. S5), and grows rapidly inward in contact with thefloor of the diencephalon. At about the :U) s stage its endreaches nearly to the infundibulum (Fig. N7). At fir
. The development of the chick; an introduction to embryology . ) /-■//■- Iu;. SS. — Optical loiiKitudiiial section of the head uf an embryo of 39 )breviati()ns as before. The hypophysis should be mentioned here, although it is notembryologically a part of the brain. It arises as a median tubu-lar invagination of the ectoderm of the ventral surface of thehead immediately in front of the oral plate at about the 20 sstage (Fig. S5), and grows rapidly inward in contact with thefloor of the diencephalon. At about the :U) s stage its endreaches nearly to the infundibulum (Fig. N7). At first ])art ofits wall is formed by the oral plate, and when this ruj^tures theeffect is to shorten the apparent length of the hypophysis (). At about the .^6 s stage its distal poilion flattens laterally FROM TWELVE TO THIRTY-SIX SOMITES 155 and shows indication of branching. Subsequently it becomesmuch branched and quite massive and unites with the infun-(Ubulum to form the pituitary body. (See Chap. VIII.) The Mesencephalon. This portion of the brain come
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