DEVELOPMENT OF THE VASCULAR SYSTEM 



505 



hyaline cell. The third and fourth varieties together constitute from 20 to 30 

 per cent, of the colorless corpuscles, but of these two varieties the lymphocytes 

 are by far the more numerous. Leucocytes having in their protoplasm granules 

 which stain with basic dyes (basophil) have been described as occurring in human 

 blood, but they are rarely found except in disease. 



The colorless corpuscles are very various in shape in living blood (Fig. 455), 

 because many of them have the power of constantly changing their form by pro- 

 truding finger-shaped or filamentous processes of their substance, by which they 

 move and take up granules from the surrounding medium. In locomotion the 

 corpuscle pushes out a process of its substance a pseudopodium, as it is called 



FIG. 455. Human colorless blood corpuscle, showing its successive changes of outline within ten minutes when kept 



moist on a warm stage. (Schofield.) 



and then shifts the rest of the body into it. In the same way when any granule 

 or particle comes in its way the corpuscle wraps a pseudopodium around it, and then 

 withdraws the pseudopodium with the contained particle into its own substance. 

 By means of these ameboid properties the cells have the power of wandering 

 or emigrating from the bloodvessels by penetrating their walls and thus finding 

 their way into the extravascular spaces. A chemical investigation of the proto- 

 plasm of the leucocytes shows the presence of nucleoprotein and of a globulin. 

 The occurrence of small amounts of fat, lecithin, and glycogen may also be 

 demonstrated. 



The blood platelets (Fig. 456) are discoid or irregularly shaped, colorless, refractile 

 bodies, much smaller than the red corpuscles. Each contains a central chromatin 

 mass resembling a nucleus. Blood platelets possess the power of ameboid move- 

 ment. When blood is shed they rapidly disintegrate 

 and form granular masses, setting free prothrorabin 

 and the substance called by Howell thromboplastin. 

 It is doubtful whether they exist normally in circu- 

 lating blood. 



DEVELOPMENT OF THE VASCULAR SYSTEM. 



Bloodvessels first make their appearance in sev- 

 eral scattered vascular areas which are developed 

 simultaneously between the entoderm and the meso- 

 derm of the yolk-sac, i. e., outside the body of the 

 embryo. Here a new type of cell, the angioblast 

 or vasofbrmative cell, is differentiated from the 

 mesoderm. These cells as they divide form small, 

 dense syncytial masses which soon join with similar 



masses by means of fine processes to form plexuses. These plexuses increase 

 both by division and growth of its cells and by the addition of new angioblasts 

 which differentiate from the mesoderm. Within these solid plexuses and also 

 within the isolated masses of angioblasts vacuoles appear through liquefaction of 

 the central part of the syncytium into plasma. The lumen of the bloodvessels 

 thus formed is probably intracellular. The flattened cells at the periphery form 

 the endothelium. The nucleated red blood corpuscles develop either from small 

 masses of the original angioblast left attached to the inner wall of the lumen or 

 directly from the flat endothelial cells. In either case the syncytial mass thus 



' 







FIG. 456. Blood platelets. Highly 

 magnified. (After Kopsch.) 



