CHAPTER XII 
THE LATER DEVELOPMENT OF THE VASCULAR 
SYSTEM 
I. Tae Heart. (For an account of the earlier development, 
see Chapters V and VI.) 
At the stage of seventy-two hours (lig. 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 
Fic. 198. — Ventral view of Dulbus 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 Greilfrom  f.4m) the bulbo-auricular angle, marks 
Hochstetter.) ; ra : : ; 
Age Mi Bebe the line of formation of the future inter- 
Bulbus cordis. b. V., The ventricular septum (Fig. 199). 
ronserictign between bulls The Development of the External 
riculo-ventricular canal. V., Form of the Heart. We have seen that 
Ventricle. 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 
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