578 THE HUMAN EMBRYO. 



ment of -the cervical region. They have at first no communica- 

 tion with the other vessels, but towards the end of the fourth 

 week their anterior ends unite to form the median basilar artery, 

 which becomes connected with the internal carotid arteries to 

 form the circle of Willis. About the same time, a series of 

 paired segmental or intervertebral arteries arise, as branches 

 from the dorsal wall of the aorta, along the cervical and thoracic 

 regions (Fig. 245, AW), and supply the spinal cord. One, or 

 more, of the anterior pairs of these intervertebral arteries become 

 continuous with the hinder ends of the vertebral arteries (Fig. 

 245, AV), which thus acquire their connection with the aortae. In 

 the later stages, some of the intervertebral arteries further back 

 become connected in similar fashion with one another, and with 

 the vertebral artery ; and by the acquisition of these posterior 

 connections, with simultaneous loss of the older and more an- 

 terior ones, the point of origin of the vertebral artery from 

 the aorta is gradually shifted backwards to the root of the neck. 



The subclavian arteries arise as branches of the vertebral 

 arteries (Fig. 246, AS) ; but, as the fore limbs increase in size, the 

 relative proportions of the two vessels soon become reversed, and 

 the vertebral arteries appear as branches of the subclavians. 



From the sides of the dorsal aorta a series of pairs of arteries 

 arise, which supply the Wolffian bodies. The coeliac axis is from 

 the first a median artery ; it arises from the ventral wall of the 

 aorta, in the anterior thoracic region, and gradually shifts back- 

 wards, until its adult point of origin, opposite the last thoracic 

 vertebra, is attained. 



In the development of the aorta, and in that of all the other 

 arteries as well, the wall of the vessel consists at first of a single 

 layer of endothelial cells. Outside this, the layer of circular 

 muscle-fibres is formed from the surrounding mesoblast, early in 

 the third week. At a later stage a layer of connective tissue is 

 formed between the muscular and the endothelial layers, but it 

 is not clear from what source this connective tissue is derived. 

 His suggests that it is formed directly from the blood in the 

 blood-vessel itself. 



3. The Veins. 



The general arrangement, and mode of development, of the 

 veins in man is the same as in the rabbit. The most important 



