CHAPTER XII 



THE LATER DEVELOPMENT OF THE VASCULAR 



SYSTEM 



I. The Heart. (For an account of the earlier development, 



see Chapters V and VI.) 



At the stage of seventy-two hours (Fig. 198), the ventricle 

 consists of a posterior transverse portion and two short parallel 

 limbs; the right limb is continuous with the bulbus arteriosus 



from which it may be distinguished by 

 a slight constriction, and the left limb 

 with the atrium. The constriction be- 

 tween the latter is the auricular canal. 

 Between the two limbs in the interior 

 of the ventricle is a short bulbo-auricu- 

 lar septum separating the openings of 

 bulbus and atrium into the ventricle. A 

 slight groove, the interventricular sulcus, 

 that extends backwards and to the right 

 from the bulbo-auricular angle, marks 

 the line of formation of the future inter- 

 ventricular septum (Fig. 199). 



The Development of the External 

 Form of the Heart. We have seen that 

 in the process of development the heart 

 shifts backwards into the thorax. The ventricle undergoes the 

 greatest displacement, owing to its relative freedom of move- 

 ment, and thus comes to lie successively to the right of, and then 

 behind the atrium. A gradual rotation of the ventricular division 

 on its antero-posterior axis accompanies its posterior displacement; 

 and this takes place in such a way that the bulbus is transferred 

 to the mid-ventral line, where it lies between the auricles (Figs. 



199 and 200). 



The auricles arise as lateral expansions of the atrium, the 



348 



Fig. 198. — Ventral view of 

 the heart of a chick em- 

 bryo of 2.1 mm. head 

 length. (After Greil from 

 Hochstetter.) 



Atr., Atrium. B. co., 

 Bulbus cordis, b. V., The 

 constriction between bulbus 

 and ventricle. C. au. v., Au- 

 riculo-ventricular canal. V., 

 Ventricle. 



