No. 3.] A HUMAN EMBRYO TWENTY-SIX DAYS OLD. 47 1 



the otic vesicle, and gives off two distinct twigs, one extending 

 in front of the vagus nerve, and the other just in front of the 

 first cervical ; they probably represent the anterior and posterior 

 cerebellar arteries. 



The origin of the vertebral is much more anterior than in 

 the adult. His, 1 and recently, Hochstetter, 2 have shown that 

 the vertebral is in many respects a segmental artery, and the 

 condition of things in this embryo confirms this view. The 

 second cervical segmental artery ends in a T, as shown in the 

 plate. Undoubtedly the cross-piece of the T is to transfer 

 the origin of the vertebral back to the second; and so on. 

 Following the segmental branches backward, I find that be- 

 tween the fifth and sixth cervical nerves, a branch passes from 

 the segmental into the anterior extremity. Although this twig 

 arises (as far as the relation of the subclavian artery to the 

 brachial plexus is concerned) one segment too far forward, I 

 think it must represent the future subclavian. 3 From this 

 point on down to the first lumbar, the segmental arteries gradu- 

 ally diminish in size. Below the first lumbar nerve no segmental 

 branches arise from the aorta. 



Between the lateral and ventral sides of the aorta there are 

 fourteen pairs of segmental branches, which pass to the Wolffian 

 body. Each pair arises on the ventral side just opposite the ori- 

 gin of the branches which pass towards the spinal cord. The an- 

 terior branches, which are quite small, arise between the seventh 

 and eighth cervical nerves, and pass directly to the anterior 

 end of the Wolffian body. Back of this pair the branches are of 

 about equal size, the last being in front of the first lumbar nerve. 



On the ventral median line of the aorta there are two dis- 

 tinct branches, — coeliac axis and the omphalo-mesenteric. At 

 the fourth lumbar several small branches are given off from the 

 ventral side of the aorta, and break up into a capillary network 

 which extends throughout the mesentery. 



Although the branch to the stomach and liver has already all 

 its relations to these organs, as the cceliac axis in the adult, its 

 origin is far too far forward. Other embryos, however, demon- 



1 An. mensch. Em., III., S. 193. 



2 Morph. Jahrbuch, 1890. 



3 Hochstetter pictures the segmental artery below the seventh nerve as giving rise 

 to the vertebral and subclavian arteries. 



