FRO:^r 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 endocarcUal wall. The later 

 development is described in Chapter XII. 



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 19 s and joins the ventral aorta at 

 about the 24 s 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.) 



