266 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
occur in blood or lymph into leucocytes and lymphocytes) are very 
variable in shape (amoeboid) and may be uni- or polynucleate. By 
their amoeboid motions they are able to pass through the endothelial 
walls of the capillaries and to pass among the cells of the different 
tissues, hence they are often called wandering cells. They have the 
power of ingesting foreign bodies which renders them of value in 
combating pathogenic organisms; and they also aid in the absorbtion 
of fats and peptones. : 
The blood plates are very little known. Their size is less than 
that of the red corpuscles and they rapidly degenerate when drawn 
from the vessels. They are circular or elliptical in outline. 
THE BLOOD-VASCULAR SYSTEM. 
The blood-vessels include the arteries, which carry the blood from 
the heart to all parts of the body; the veins, which bring it back, and the 
Fic. 273.-—Embryonic circulation of snapping turtle, Chelydra, showing relations of 
allantois, after Agassiz and Clarke. a, right auricle; al. allantois; av, allantoic vessels; c, 
caudal vein; da, dorsal aorta; h, hypogastric artery; j, jugular; /, liver; oa, ov, omphalo- 
mesenteric artery and vein; pc, post-cardinal; sc, subcardinal vein; uv, umbilical vein; Wy 
Wolffian body; y, yolk sac. 
capillaries which connect the ends of the arteries and veins, for the 
system is closed, and there is a complete circulation. 
Since all transfer of gases and nourishment takes place through the 
‘capillaries, these vessels have extremely thin walls, consisting of a 
single layer of squamous epithelium, the so-called intima. Usually, as 
