. The development of the chick; an introduction to embryology . of the yolk the axis of the embr3o is twisted in the inter-mediate region. This twist is transferred farther and fartherbackwards as the turning of the head gradually involves thetrunk, until finalh, at about ninety-six hours, the embryo liesentirely on its left side. Exceptionally the rotation may be in the in^•erse direction(heterotaxia); in such cases it is often associated with situs in-versus viscerum. Heterotaxia has been produced experimentally(Fol and Warynsky). After the appearance of the cranial flexure a second trans-ve
. The development of the chick; an introduction to embryology . of the yolk the axis of the embr3o is twisted in the inter-mediate region. This twist is transferred farther and fartherbackwards as the turning of the head gradually involves thetrunk, until finalh, at about ninety-six hours, the embryo liesentirely on its left side. Exceptionally the rotation may be in the in^•erse direction(heterotaxia); in such cases it is often associated with situs in-versus viscerum. Heterotaxia has been produced experimentally(Fol and Warynsky). After the appearance of the cranial flexure a second trans-verse flexure appeals in the embryo, this time at about thejunction of head and trunk, hence known as the cervical ficrure(Figs. 73, 99, etc.). This flexure gradually increases in extentuntil the head forms a right, or even smaller, angle with thetiunk; thus the fore-brain is turned to such an extent that itsanterior end points backwards and its vential surface is opposetlto the ventral surface of the throat (Fig. 117). 134 THE DEVELOPMENT OF THE CHICK : Fig. 71. — Entire embryo of 16 s, drawnfrom al)ove as a transparent object. Notethe cranial flexure, also the rotation of thehead on its left side,an. P., Auditory pit. F. B., B. 1, First (Uvision of the hind F. Am., Ilead-fokl of the amnion. Hm. F.,Hyomandibular furrow. Pram., Proam-nion. M. B., Mitl-l)rain. op. Ves., Oi)ticvesicle, pr. str., Primitive streak, s 2, s 4,s 16, Second, fourth, and sixteenth o. m., omi)halo-mescntcric vein. VII-VIII,The acustico-facialis i)rini()nlium. IX-X,Primonhum of the glossopharyngeus andvajrus. Tlie entire trunk tends ulso to liend ventrally, to developa dorsal convexity, and this apjjroxiniates its jjosterior end to thetip of the head. These flexures are characteristic of amniote FROM TWELVE TO THIRTY-SIX SOMITES 135 vertebrate embryos; the cause appears to lie in the precociousdevelopment of the central nervous system, of which
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