114 THE TISSUES 



give rise to cells of its own kind, it is never capable of producing ceUs of any other 

 kind. 



Two views are held in regard to the manner in which the embryonic nucleated 

 red cell gets rid of its nucleus in becoming the non-nucleated red cell of the adult. 

 According to one the nucleus is absorbed within the cell and gradually secreted; 

 according to the other the nucleus, as a whole, is extruded. 



In early embryonic life especially active proliferation of red cells occurs in 

 the blood-vessels of the liver. This has led to the considering of the liver as a 

 blood-forming organ. The liver cells themselves, however, take no actual 

 part in the formation of blood cells, the blind pouch-like venous capillaries of 

 the liver, with their slow-moving blood currents, merely furnishing a peculiarly 

 suitable place for cellular proliferation. Before birth the splenic pulp and bone 

 marrow become blood-forming organs. In the adult the bone marrow is prob- 

 ably, under normal conditions, the main if not the sole seat of red-cell formation. 



During foetal life the number of nucleated red cells constantly diminishes, 

 while the number of non-nucleated red cells increases. At birth there are 

 usually but few nucleated red cells in the general circulation, although even in 

 the adult they are always found in the red bone-marrow. 



The origin of the blood platelets is not known. They have been described 

 as originating in the extruded nuclei of the red cells, as disintegrating leucocyte 

 products, as red cells in process of development, as red cells in process of disin- 

 tegration, as albuminoid precipitates, as a specific blood cell. According to 

 Wright,^ they are derived from the megakaryocytes of bone marrow and 

 other blood-forming organs. 



TECHNIC 



(i) Fresh Blood. — Prick a finger with a sterile needle. Touch the drop of 

 blood to the centre of the slide and cover quickly. For immediate examination 

 of fresh blood no further preparation is necessary. Evaporation may be pre- 

 vented by cementing, or by smearing a rim of vaseline around the cover-glass. 



(2) Blood Smears. — From the same or a second prick take up a drop of 

 blood along the edge of a mounting slide. Quickly place the edge against the 

 surface of a second slide and draw the edge across the surface in such a manner as 

 to leave a thin film or smear of blood. Allow the smear to become perfectly dry 

 and stain by technic 11, p. 32. By this method the acidophile granules are stained 

 red, basophile granules purple, and neutrophile granules a reddish-violet. 



Good results may also be obtained by fixing the dried smear for half an hour 

 in equal paits alcohol and ether and staining first in a strong alcoholic solution 

 of eosin, then in a rather weak aqueous solution of methylene blue. 



^ Jour. Morpli., xxi, igio. 



