L K PI I )()SI H EN VX II A DOXA . 
119 
Posterior Cerebral Veins. — At Stage 28 a small 
vessel from the brain opens into each anterior cardinal vein 
some distance posterior to the otocysts. A little later 
(Stage 31+) the posterior anastomosis of the lateral cephalic 
vein with the anterior cardinal occurs just in this region, 
and, when the median cephalic part of the latter vessel 
atrophies, the cerebral vein (Text-fig. 24 e, P. G.) and the 
short persistent posterior part of the median cephalic 
open together into the lateral cephalic vein. This relation- 
ship persists in the adult, the cerebral vessel forming the 
posterior cerebral branch of the anterior cardinal vein. 
Lateral Vitelline and Sn bintestinal Veins. — When 
the primitive vitelline veins become connected over the 
lateral surfaces of the yolk with the anterior and posterior 
cardinals so that they may now be termed the ducts of 
Cuvier (Text-fig. 24 c, P. C.), they also become connected 
with the general vitelline meshwork by means of two main 
lateral vessels one on either side (Text-fig. 24 c, r. I. 
V. F,). These lateral vitelline veins extend backwards and 
ventrally over the sides of the yolk towards the anus, their 
tributary vessels forming intricate anastomoses all over its 
surface : in the region of the anus they fuse and form the, at 
first extremely short, subintestinal vein (Text-fig. 26 A, lat, 
V. F, Si. F). 
A little later (Stage 32) three main vessels can be traced 
over the yolk towards the heart, viz. two lateral and one 
smaller median vessel (Text-fig. 26 b, r. and 1. lat. V. V., Si. F). 
34ie appearance of this median vessel by short-circuiting* 
through the network on the yolk, synchronises with the 
development of blood-sinuses in the liver (Stage 31), which 
form a wide vessel, opening ventrally into the sinus venosus 
on the right side. The median vitelline or subintestinal vein 
is continuous with the liver-sinuses over the antero-ventral 
surface of the yolk, the former channel thus obtaining 
access to the heart. With the further development of 
the embryo, and the definition of the liver and gut, the 
right lateral vitelline vein disappears, and the left vessel 
