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-tw^o hours shows 

 a thin myocardial and a thick endocardial 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 primitive vascular system of vertebrate embryos is a 

 capillary network in all parts of the blastoderm and of the 

 embryo. Main trunks arise by development of parts of the 

 network corresponding to the rate and direction of embryonic 

 growth and thus answering to the vascular needs of growth. 

 The vascular system forms at all stages a continuous endothelial 

 tree whose primitive form in all parts is a capillary network. 

 This idea, which we owe originally to Aeby, has been worked out 

 in a masterly way by H. M. Evans. (See lit. Chap. V.) 



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 

 da^'S. (See Chap. XII for account of the fifth and sixth arches.) 



