DEVELOPMENT OF THE CHICK SECOND PEEIOD. 319 



Immediately under the head of the embryo, three blood-red 

 bounding points are seen (fig. 346, d), the expression of the 

 alternating contractions of the three divisions of the heart, 

 which are now in the course of formation, the sinus venosus 

 (fig. 339 A", 340 /), which receives the veins, and towards the 

 end of the third day shows traces of the two auricles, the 

 ventricle (339 i, 340 m), and the bulbus aorta (339 /, 340 ri), 

 divided from the ventricle by a contraction. In this period 

 the heart presents such diversities that it may be said to be in 

 a state of ceaseless metamorphosis, both as regards form and 

 position. On the second day, it is a somewhat spirally 

 twisted canal lying under the brain (fig. 339, i) ; on the 

 third day, it has drawn itself more backwards, become more 

 concentrated, and bent round, as it were, into a kind of loop 

 (fig. 340, m\ when it appears to project in the form of a tu- 

 mour between the ventral laminse (figs. 340 m, and 341 h], 

 first inclining to the left and then to the right, and being 

 all the while within the compass of the involucrum capitis 

 (fig. 347, /). The ventricle, which during the third day is 

 still canalicular, becomes more globular on the fourth day 

 (fig. 345, h\ and pointed underneath, so that it acquires the 

 proper heart-shape (fig. 342, B, g] ; it then lies very much 

 to the right, whilst the sinus venosus, which is become more 

 distinct from it, lies more to the left (fig. 345, behind K). 

 At the end of the third day, the constriction between the 

 ventricle and aortal bulb is already well marked (fig, 340, n). 

 On the fourth day, the muscular mass of the heart and the 

 septum ventriculorum is produced ; in the sinus venosus the 

 septum is not begun to be formed till the fifth day, and the 

 two apices into which the veins even on the third day were 

 seen to plunge (fig. 340, below I}, enlarge, and become the 

 auricles. Some time before the bulbus aortse becomes distinctly 

 pinched off (fig. 347, /), it divides at the beginning of the third 

 day into four pairs of vascular arches, which show themselves 

 through the abdominal laminae, the most posterior of the four 

 being the smallest (fig. 347, 1 4) ; after the formation of the 

 branchial fissures they He behind the sickle-shaped branchial 

 arches (figs. 339, 340, 343, B) ; they unite on either side 

 upon the vertebral column into an aortal root ; the two 

 roots blend more posteriorly, and form the common aorta 

 (fig. 347, h). The vascular arches undergo considerable 



