190 ABTHUE B. SHIPLEV". 



tomose with one another in a very complex manner. They 

 contain numerous nuclei, and show a longitudinal striation 

 though not a transverse one. The centre of the ventricle is 

 comparatively free from them, but at the sides they form a 

 spongy reticulum in the meshes of which corpuscles abound. 



The ventricle passes anteriorly into the ventral aorta, and 

 at the point where the aorta passes into the solid tissue 

 between the gills there is another pair of valves resembling 

 the auriculo-ventricular ones. The ventral aorta, like the 

 other vessels, arises by a split in the mesoblast which subse- 

 quently acquires a definite wall. It passes forward as a single 

 vessel in the ventral median line until it reaches the thyroid 

 gland, and here it splits in two branches. Each branch then 

 passes forward on one side of this body, and ends in the most 

 anterior gill vessel. From the single part of the ventral aorta 

 three pairs of vessels are given off, passing in front of the 

 fifth, sixth, and seventh gill-slits respectively. The posterior 

 wall of the seventh cleft bears no gill filaments, and has no 

 vessel. From each side of the double part of the ventral aorta 

 five vessels are given off, the four posterior of these pass in 

 front of the first, second, third, and fourth gill-slits. The 

 most anterior is the vessel which in the earlier stages passes in 

 front of a gill-slit which subsequently disappears. In the 

 older embryos, when the mouth is fully formed it runs along 

 the base of the velum. 



The vessels after traversing the gills unite in the dorsal 

 middle line to form the dorsal aorta ; this runs backward to 

 the posterior end of the body, lying just underneath the noto- 

 chord. From its first appearance it gives off two transverse 

 vessels in the neighbourhood of the pronephros ; these supply 

 the glomerulus. Anteriorly it gives off a pair of vessels to 

 supply the upper lip, the carotids. In the older larvae the 

 aorta gives off a vessel which passes dorsally up one myotome, 

 then along the dorsal surface of the myomere behind it, and 

 hence the blood" is collected by a vein which returns it to the 

 posterior cardinal down the next myomere. The larvae are 

 fairly transparent, and in each myotome these two opposite 



