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 absorb tion 

 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 



FIG. 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; w, 

 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 



