II2 NORMAL HISTOLOGY. 



of a few granules of common salt being advantageous in the case 

 of old clots), on slightly heating until bubbles appear, numbers of 

 dark-brown irregular rhombic prisms form. These are the hsemin 

 crystals of Teichmann, which are positive indications of the 

 presence of blood, but have no value in the determination of its 

 source. They vary greatly in size and considerably in form, the 

 peculiar unequally-notched ends presented by the larger crystals 

 being quite characteristic. 



DEVELOPMENT OF THE BLOOD-CORPUSCLES. 



The origin of the colorless blood-cells must be referred to 

 the lymphoid tissues, since these elements are identical with those 

 occurring within the lymph with which they are poured into the 

 blood-current. The colorless corpuscles appear later than the red 

 cells, the first ones probably entering the circulation as migratory 

 mesodermic elements. The lymphatic or adenoid tissues, however, 

 undoubtedly constitute the principal sources of the colorless blood- 

 corpuscles, which are produced by the division of the numberless 

 masses of active protoplasm contained within the various aggrega- 

 tions of lymphoid tissue throughout the body. 



The multiplication of existing colorless cells which takes place 

 normally, but which is especially active under the stimulus of patho- 

 logical conditions, accounts for the origin of a certain number of 

 white corpuscles ; the division of the cellular elements of connective 

 tissue is regarded by some as an additional source of these blood- 

 cells. The efferent lymph-streams passing from the lymphatic tissue, 

 as well as the blood contained in the splenic vein, are richer in color- 

 less cells than are the corresponding afferent currents, showing that 

 the augmentation is due to the new elements contributed by the 

 lymphoid tissues through which the currents pass. 



The origin of the colored blood-cells is usually considered as 

 taking place during two epochs before and after birth. It must be 

 remembered, however, that such division is conventional and largely 

 arbitrary, since the period at which the primary embryonic processes 

 of such formation cease and are replaced by those maintained 

 throughout life is uncertain and variable ; in man and mammals 

 born in a condition of advanced development the production of 

 blood-corpuscles within the marrow is instituted before the termina- 

 tion of intra-uterine life. 



Before Birth. The first blood-cells originate outside the body 

 of the embryo, within the angioblastic cells of the mesodermic 

 tract of the vascular area. Certain cells of this layer increase in 

 size and undergo proliferation of their nuclei, forming multinucleated 

 areas known as the blood-islands of Pander. These subsequently 



