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 

 Fig. 198. — Ventral view of bulbus and atrium into the ventricle. A 

 the heart of a chick em- slight groove, the interventricular sulcus, 

 bryo of 2.1 mm. head that extends backwards and to the right 

 length. (After Greil from ^^.^^ ^^le bulbo-auricular angle, marks 



. , ...' -D the line of formation of the future inter- 



Atr., Atrmm. B. co., 



Bulbus cordis, b. V., The ventricular septum (Fig. 199). 



andTJntric'le^^^raT v"! A^^ ^^^ Development of the External 



riculo-ventricular canal. V., Form of the Heart. We have seen that 

 Ventricle. -j,^ ^1^^ 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 



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