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 signioid 

 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 



v.v- 



VfCC 



Figs. 23 and 24. Anterior portions of chick-embryos of the 38th and 48th 

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

 DUVAL.) 



ao. Right and left aortse. au. Auditory involution, c. 2 . Ventricular por'i -n of 

 heart. c 3 . Auricular portion of heart, e. Eye. h. Heart, op. Priman ^tic 

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

 hind-brain, t.a. Truncus arteriosus. v.a. Vitelline arteries. v,v. Vitelline veni. 

 i, 2,3, Transitory gill-slits. 



that represented in Fig. 23 the heart was pt ! ",?ctly 

 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 



