52 



ANATOMY OF AMPHIOXUS. 



the embryonic development proceeds, this simple tubular 

 heart widens out, acquires a series of constrictions, and 

 undergoes a remarkable flexure known as the sigmoid 

 flexure. Two stages in the formation of the sigmoid 

 flexure of the heart of the chick-embryo are shown in 

 Figs. 23 and 24. At a somewhat earlier stage than 



\^yphl 



Figs. 23 and 24. — Anterior portions of cliick-embryos of the 3Sth and 48th 

 hour of incubation, seen from below, to illustrate formation of heart. (After 

 Duval.) 



ao. Right and left aortas, au. Auditory involution, c.y Ventricular portion of 

 heart, c^. Auricular portion of heart, e. Eye. //. Heart, op. Primary optic 

 vesicle, p.f.b- Primary fore-brain, p.m.b. Primary mid-brain, p.h.b. Primary 

 hind-brain, t.a. Truncus arteriosus, v.a. Vitelline arteries, v.v. \'itelline veins. 

 /, 2, j>, Transitory gill-slits. 



that represented in Fig. 23 the heart was perfectly 

 straight. In this figure it is still a simple dilated tube, 

 but no longer straight. It has become bent outwards 

 into a U-shape. At the stage of Fig. 24 well-marked 

 constrictions (the indications of the later division into 

 auricle and ventricle, etc.) have appeared in the heart, and 

 the simple U-shaped flexure of the latter has become 



