FROM TWELVE TO THIRTY-SIX SOMITES 203 
myocardium becomes more or less filled by multiplication of 
the endocardial cells. On the side of the myocardium there is 
first a thickening, and then anastomosing processes are sent out 
towards the endocardium. Cavities also arise within the thick- 
ened myocardium and all communicate. The endocardial cells 
then form a covering to all myocardial processes and cavities, 
and the cavities thus lined communicate with the main endo- 
cardial cavity. Thus the wall of the ventricles becomes spongy 
and all the cavities in it are lined by a layer of endocardium 
and communicate with the endocardial cavity. In the bulbus 
finally there is a great thickening of the endocardium produced 
by multiplication of its cells, but no corresponding change in 
the myocardium; thus the bulbus at seventy-two hours shows 
a thin myocardial and a thick endocardial wall. The later 
development is described in Chapter XIT. 
The Arterial System. The description of the development of 
the arterial system proceeds from the stage of 12 somites described 
in the last chapter. 
The following should be added to the account there given. At this 
stage Kastschenko finds three pairs of small arterial vessels in front of 
the first visceral pouch running from the dorsal towards the ventral 
aorta, which, however, they do not meet. At about forty-six hours 
the first two of these have disappeared. The third, however, has become 
almost as large as the hyoid aortic arch. Kastschenko thinks it prob- 
able that this is the true mandibular arch. Though he did not find it 
in connection with the ventral aorta, he thinks it may form such a union 
of short duration. I have actually found such a vessel joining the man- 
dibular arch to the dorsal aorta in an embryo of 35 somites. On the 
other hand, what we have previously called the mandibular arch may 
be the true one displaced in the course of phylogeny. 
The Aortic Arches. An arch of the aorta is formed in each vis- 
ceral arch; they arise successively as buds from the roots of the dor- 
sal aorta in the order and time of formation of the visceral arches. 
Thus the first or mandibular aortic arch is formed at the stage of 
9-10 somites; the second or hyoid aortic arch arises from the dor- 
sal aorta at about the stage of 19s and joins the ventral aorta at 
about the 24s stage. The third is completely formed at the stage 
of 26 somites. The fourth is completely formed at the stage of 36 
somites; and the fifth and sixth arise during the fourth and fifth 
days. (See Chap. XII for account of the fifth and sixth arches.) 
