. The development of the chick; an introduction to embryology . gt;ulbus is transferredto the mid-ventral line, where it lies between the auricles ( and 200). The auricles arise as lateral cx])ansions of tlie atrium, the 348 Iu;. li)8. — Ventral view ofthe heart of a chick em-bryo of mm. headlength. (After Greil fromHochstetter.)Atr., Atrium. B. co.,Bulbus cordis, b. V., Theconstriction between bullnisand ventricle. ,Au-riculo-ventricular canal. V.,Ventricle. LATER DEVELOPMENT OF VASCULAR SYSTEM 349 left one first at an early stage and the right one later. The leftauricle is
. The development of the chick; an introduction to embryology . gt;ulbus is transferredto the mid-ventral line, where it lies between the auricles ( and 200). The auricles arise as lateral cx])ansions of tlie atrium, the 348 Iu;. li)8. — Ventral view ofthe heart of a chick em-bryo of mm. headlength. (After Greil fromHochstetter.)Atr., Atrium. B. co.,Bulbus cordis, b. V., Theconstriction between bullnisand ventricle. ,Au-riculo-ventricular canal. V.,Ventricle. LATER DEVELOPMENT OF VASCULAR SYSTEM 349 left one first at an early stage and the right one later. The leftauricle is thus larger than the right for a considerable period oftime in the early development. When the right auricle growsout it passes above the bulbus, which is already in process ofrotation, and the two auricles then expand ventrally on eachside of the bulbus. The apex of the ventricle belongs primarilyto the left side and this remains obvious as long as the externalinterventricular groove exists. In the adult the apex of theheart belongs to the left Fig. 199. — Ventral view of the heart of achick embryo of 5 mm. liead-length.(After Masius.)Atr. d., s., Right and left auricles. B. Co. BulJaus cordis. V. Ventricle. The varying positions occupied by the chambers of the heart in rela-tion to the body axes constitute a serious difficulty in describing thedevelopment. For instance, the auricular canal is at first in front ofthe atrium (before any bending of the heart takes place). As the ven-tricular loop turns backward and beneath the atrium, the auricularcanal is ventral to the atrium; and finally, as the ventricles assume theirdefinitive position behind the auricles, the derivatives of the auricularcanal (auriculo-ventricular openings) come to lie behind the atrium. Inother words, the atrium rotates around a transverse axis through nearly180 degrees in such a way that its original anterior end becomes succes- 350 THE DEVELOPMENT OF THE CHICK sively venti-al and poster
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