. The development of the chick; an introduction to embryology . come deeper, owing to the forward extension of the headabove the blastoderm. The posterior extension of the splanch-nopleuric head-fold lengthens the fiooi- of the fore-gut; it iscaused by the median growth and concrescence of folds of thesplanchnoplenre (Fig. ). Along with this process is involvedthe development of tlie heart (lescril)ed faither on. The growthin length of the fore-gut may be realized l)y a c()nij)arison ofFigs. 50, 52, 62, etc. Tluis by the 12 s stage a considerable section of the fore-gutis already establishe
. The development of the chick; an introduction to embryology . come deeper, owing to the forward extension of the headabove the blastoderm. The posterior extension of the splanch-nopleuric head-fold lengthens the fiooi- of the fore-gut; it iscaused by the median growth and concrescence of folds of thesplanchnoplenre (Fig. ). Along with this process is involvedthe development of tlie heart (lescril)ed faither on. The growthin length of the fore-gut may be realized l)y a c()nij)arison ofFigs. 50, 52, 62, etc. Tluis 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 veiy Hat cells. The lateral extensions may l)e regartled 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. -fr^r. Fig. -tS. — Stage of first intcrsomitic groovedrawn from an entire mount in balsam bytransmitted light. a. (. v., Amnio-cardiac vesicle, a. o., In-ner margin of Area opaea-. Fct., , Fntoderm H. F., Head-fold. ,First intersomitic furrow, med. Medullaryj)late. Mes., Mesoderm, n. Neural groove,pr. gr., Irimitive gr()o\(\ Iia, jiioanmion. in the next chapter. At the 12 s stage one such place of contactis already formed, lying a sliort distance in front of the thickenedectoderm destined to form the auditory j^it. HEAD-FOLD TO TWELVE SOMITES 95 Andther 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 paiietal cavities meet posterior to the oral ])late (Figs. 67and 75). Transverse sections show the oral plate to be depressedbeneath the level of the ventral surface
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