. The development of the chick; an introduction to embryology . e deeper, owing to the forward extension of the headabove the blastoderm. The posterior extension of the splanch-nopleuric head-fold lengthens the floor of the fore-gut; it iscaused by the median growth and concrescence of folds of thesplanchnopleure (Fig. 53). Along with this process is involvedthe development of the heart described farther on. The growthin length of the fore-gut may be realized by a comparison ofFigs. 50, 52, 62, etc. Thus by the 12 s stage a considerable section of the fore-gutis already established (Fig. 63);
. The development of the chick; an introduction to embryology . e deeper, owing to the forward extension of the headabove the blastoderm. The posterior extension of the splanch-nopleuric head-fold lengthens the floor of the fore-gut; it iscaused by the median growth and concrescence of folds of thesplanchnopleure (Fig. 53). Along with this process is involvedthe development of the heart described farther on. The growthin length of the fore-gut may be realized by a comparison ofFigs. 50, 52, 62, etc. Thus by the 12 s stage a considerable section of the fore-gutis already established (Fig. 63); this is the pharyngeal division;from the first it is extremely broad, and lunate in cross-section(Fig. 54), the floor being composed of columnar cells, and the roof 94 THE DEVELOPMENT OF THE CHICK of very flat cells. The lateral extensions may be regarded asdiverticula; subsequently these grow more rapidly at four placesalong their length, and come in contact with the ectoderm. Thusfour pouches are established on each side as described in detail & Fig. 48. — Stage of first intersomitic groovedrawn from an entire mount in balsam bytransmitted light,a. c. v., Amnio-cardiac vesicle, a. o., In-ner margin of Area opaca. Ect., , Entoderm H. F., Head-fold. i. ,First intersomitic furrow, med. pi., Medullaryplate. Mes., Mesoderm, n. gr., Neural groove,pr. gr., Primitive groove. Pra, proamnion. in the next chapter. At the 12 s stage one such place of contactis already formed, lying a short distance in front of the thickenedectoderm destined to form the auditory pit. HEAD-FOLD TO TWELVE SOMITES 95 Another place of fusion between the fore-gut and the ecto-derm is the so-called oral plate (pharyngeal membrane), whichoccupies a mid-ventral position at the extreme anterior parietal cavities meet posterior to the oral plate (Figs. 67and 75). Transverse sections show the oral plate to be depressedbeneath the level of the ventral surface of the head at the s
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Keywords: ., bookauthorlilliefrankrattray187, bookcentury1900, bookdecade1910
