686 HUMAN ANATOMY. 



or thrombocytes. As they are extraordinarily sensitive to exposure, even to entire 

 disappearance, special precautions are necessary to insure their presence in an unal- 

 tered condition in preparations examined. If blood be drawn directly into and mixed 

 with a drop of .85 per cent, salt solution, or, still better, into one of weak osmic acid 

 solution, the blood plaques appear as round or oval discs, from .002-.004 mm. in 

 diameter, usually somewhat less than one-third of the size of the red cells. From 

 these they further differ in being colorless and devoid of hemoglobin and in staining 

 readily in very dilute solutions of methyl-violet. The blood plaques appear faintly 

 granular and contain masses of chromatic substance representing a nucleus. They 

 seem to be minute cells and are capable of undergoing amoeboid movement. They 

 possess the ability of rapidly throwing out processes and adhering together on 

 coming into contact with foreign bodies. Their assumed role, that of arresting 

 hemorrhage by assisting in the formation of a coagulum, suggested the name, 

 thrombocytes, given them by Dekhuygen. Notwithstanding the attention bestowed 

 upon them, the source of the plaques is still undetermined. This has been x-ariously 



attributed to disintegration of the leucocytes, 

 Fig. 649. to extrusion from the red cells, or from the 



megakaryocytes, or to destruction of the en- 

 dothelial lining of the vessels. None of these 

 f'\ ^'- assumptions can be regarded as established, 



or even probable, in view of their constant 



presence and large normal quota— ^an average 



of 300,000 plaques in one cubic millimeter of 



blood. 



Granules. — In addition to the corpus- 

 cles and the plaques, extremely minute gran- 

 ®® ^ ules occur in varying numbers in normal 



© human blood. The nature of these particles 



differs. Some are undoubtedly finely divided 

 _/ fat; others, described by H. F. Miiller under 



"^-^-"rf.^® the name, hemoconia, are of uncertain compo- 



sition, but not fatty; while a certain propor- 



Human blood, showing red cells and ,• . 111 1 • 1 r 1 ^• • • 



blood plaques. X 625. tion IS probably derived from the dismtegration 



of endothelial and blood-cells. The destruc- 

 tion of the latter is accountable for the minute particles of pigment that are constant, 

 if not numerous, constituents of the circulation. 



DEVELOPMENT OF THE BLOOD-VESSELS AND CORPUSCLES. 



The earliest blood-vessels appear within the extra-embryonic mesoblast covering 

 the vitelline sac and, therefore, beyond the limits of the embryo proper and entirely 

 independent of the heart and axial trunks. In the lower mammals, the formation 

 of the primary vessels takes place towards the periphery of a limited field, known as 

 the vascular area, that encircles only a portion of the vitelline sac; in man the 

 limited proportions of the latter enable the net-work of developing blood-channels to 

 extend completely over the vesicle, so that the vascular area becomes coextensive 

 with the yolk sac. Although the initial stages in the formation of the primary 

 blood-vessels have never been observed in man, since the vessels were already 

 present over the vitelline sac in the youngest embryo so far examined, it is probable 

 that the development of the human vascular tissues is essentially the same as that 

 seen in other mammals. 



In the rabbit, the first indications of the developing blood-vessels are cords or 

 groups of spherical cells that appear within the deeper, later splanchnic, layer of the 

 mesoblast covering the vitelline sac. These tracts become larger in consequence 

 not only of prolifera'tion, but also of separation of the component cells. The meso- 

 blastic elements surrounding the tracts soon become disposed as enclosing walls, 

 within which the separated cells, now suspended in a clear fluid that has meanwhile 

 appeared, represent the earliest blood-cells. 



The channels thus established unite into a net-work of primary blood-vessels that 

 at first occupies the periphery of the vascular area, but later extends towards the 



