SUCCESSIVE FORMS OF BLOOD-CELLS. 115 



organisms die ! Physiology has its passing wonders as well as astron- 

 omy. 



In the life of man there are three periods distinguished from each other 

 by the nature or structure of the blood-cells. Those of the Successivc 

 first period originate simultaneously with, or even previously races of blood- 

 to, the heart. These are sometimes designated as embryo ^ 

 cells, and in that view bear the same relation to those of the second pe- 

 riod as do the lymph corpuscles to those of the third. They are color- 

 less and spherical cells, containing granules of fatty material, and having 

 a central nucleus. These are developed, by a process of internal deli- 

 quescence, into cells of the second period, which have acquired a red col- 

 or, and in oviparous vertebrates an elliptical form, though in man they 

 are circular. They are flat or disc-like in shape, have a diameter of 

 about 2-^-Q of an inch, with a central nucleus of half that size. Some- 

 times they appear to undergo multiplication by division of the nucleus. 



These cells of the second period are replaced by those of the third, the 

 transition being clearly connected with the production of lymph and chyle 

 corpuscles. By the end of the second month of foetal existence the re- 

 placement is complete, and the class of cells or discs that has now arisen 

 is continued during life. The mode of their production, according to Mr. 

 Paget, is this. The chyle or lymph corpuscle loses its granular aspect, 

 and acquires a pale red color, which gradually deepens ; the corpuscle be- 

 comes smooth, loses its spherical form, and, condensing, takes on a con- 

 vex lenticular shape, and eventually a bi-concave. While this change 

 of structure is going on, the specific gravity increases through the con- 

 densation, and the development closes by the spherical, white, granular, 

 lymph corpuscle becoming a red, bi-concave, non-nucleated, circular, small, 

 and heavy blood disc. 



The cell of the first period is therefore spherical, white, and nucleated ; 

 that of the second, red, disc-shaped, and nucleated ; that of the third, red, 

 disc-shaped, bi-concave, and non-nucleated. 



The primordial cell advances in development to different points in dif- 

 ferent orders of living beings. The blood of invertebrated Development 

 animals contains coarse granule cells, which pass forward to ^th^atiS 

 the condition of the fine granule cells, and reach the utmost series, 

 perfection they are there to attain in the colorless nucleated cell of the 

 first period of man. In oviparous vertebrated animals the development 

 is carried a step farther, the red nucleated cell arising, and in them it 

 stops at this, the second period. In mammals the third stage is reached 

 in the red, non-nucleated disc, which is therefore the most perfect form. 



This perfect form of blood cell, as it occurs in man, may be described 

 as presenting a flattened shape ; the bright spot, which is sometimes seen 

 in the centre, arising from a refraction of light due to the form of the 



