THE CIRCULATING LIQUIDS OF THE BODY 31 



creased when digestion is going on. The number of blood- 

 plates is about 300,000 to the cubic millimetre of blood. 



Life-history of the Corpuscles. The corpuscles of the blood, 

 like the body itself, fulfil the allotted round of life, and then 

 die. They arise, perform their functions for a time, and 

 disappear. But although the place and mode of their origin, 

 the seat of their destruction or decay, and the average 

 length of their life, have been the subject of active research 

 and still more active discussion for many years, much yet 

 remains unsettled. 



In the embryo the red corpuscles, even of those forms 

 (mammals) which have non-nucleated corpuscles in adult 

 life, are at first possessed of nuclei. In the human foetus, 

 at the fourth week all the red corpuscles are nucleated. 



FIG. 4. CURVE SHOWING PROPORTION OF WHITE CORPUSCLES TO RED AT 

 DIFFERENT TIMES OF THE DAY (AFTER THE RESULTS OF HIRT). 



At I the morning meal was taken; at II the mid-day meal; at III the evening 

 meal. During active digestion the number of lymphocytes in the blood is greatly 

 increased, both absolutely and relatively to the number of the other leucocytes. 



Later on the nucleated corpuscles gradually diminish in 

 number, and at birth they have almost or altogether dis- 

 appeared, some of them, at least, having been converted by 

 a shrivelling of the nucleus into the ordinary non-nucleated 

 form. In the newly-born rat, which comes into the world 

 in a comparatively immature state, many of the red 

 corpuscles may be seen to be still nucleated. The first 

 corpuscles formed in embryonic life are developed outside of 

 the embryo altogether (in the guinea-pig). Even before the 

 heart has as yet begun to beat, certain cells of the mesoblast 

 (see Chap. XIV.) in a zone (' vascular area ') around the 

 growing embryo begin to sprout into long, anastomosing 

 processes, which afterwards become hollowed out to form 



