THE SOMITES OF THE CHICK 69 



tube and the unsegmented mesoderm of the head and backward 

 between the former and the somites. Both of these strands are 

 quicklj' replaced by blood-vessels; the former in an embrj^o of 

 eight segments (H. E. C. no. 642) is transformed into the first 

 intersegmental artery ; and the latter forms the posterior end of the 

 vena capitis medialis and, behind the first intersegmental cleft, 

 the beginning of a chain of anastomoses between the distal ends 

 of the intersegmental arteries. An apparently isolated vessel 

 represents the beginning of the third or oblique portion of the 

 anterior cardinal vein. 



In an embryo of fourteen segments, each aorta bears a dorso- 

 lateral branch in the first intersegmental cleft. One of these is 

 connected with a small and somewhat tortuous transverse vessel 

 which, joining the posterior end of the vena capitis jnedialis and 

 the anterior end of the oblique portion of the anterior cardinal 

 vein, forms the transverse portion of the latter vessel. On the 

 other side of the embryo, the posterior end of the vena capitis 

 medialis and the distal end of the dorso-lateral branch of the aorta 

 are bound together only by strands of vascular cells. Similar 

 dorso-lateral branches of the aorta occur in the second interseg- 

 mental cleft and are connected with the intersegmental arteries 

 by transverse vessels. 



The anterior cardinal veins of two embryos of fifteen segments 

 in the Harvard embryological collection (nos. 1444 and 1460) 

 are in the main like those of the embryo of the same number of 

 segments figured and described by Evans. The transverse por- 

 tion of this vein in both embryos is connected with the aorta by a 

 quite direct vessel or in one case by two vessels. Both embryos 

 are probably younger than the one studied by Evans for in one 

 (no. 1460) the common cardinal veins are not present, and in the 

 other they are very minute. The second intersegmental cleft 

 on each side contains a single T-shaped branch of the aorta which 

 is apparently formed by the fusion of the preceding vessels. One 

 arm of the T anastomoses with the posterior prolongation of the 

 vena capitis medialis, the other extends upward between the stalks 

 of the second and third somites and then expands in an irregular 

 flattened vessel which, uniting by minute anastomoses with simi- 



