20 



A MANUAL OF PHYSIOLOGY 



In the embryo the red corpuscles, even of those forms (mam- 

 mals) which have non-nucleated corpuscles in adult life, are at 

 first possessed of nuclei, and approximately spherical in form. 

 In the human foetus, at the fourth week all the red corpuscles 

 are nucleated. Later on the nucleated corpuscles gradually 

 diminish in number, and at birth they have almost or altogether 

 disappeared, 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. 



Even before the heart has as yet 

 begun to beat, certain cells of 

 the mesoderm (see Chap. XIV.) 

 in a zone (' vascular area ') 

 around the growing embryo begin 

 to sprout into long, anasto- 

 mosing processes, which after- 

 wards become hollowed out to 

 form capillary bloodvessels. At 

 the same time clumps of nuclei, 

 formed by division of the original 

 nuclei of the cells, gather at the 

 nodes of the network. Around 

 each nucleus clings a little lump 

 of protoplasm, which soon de- 

 velops haemoglobin in its sub- 

 stance ; and the new-made cor- 

 puscles float away within the 

 new-made vessels, where they 

 rapidly multiply by mitosis. In 

 later embryonic life the nucleated corpuscles continue in part 

 to be developed within the bloodvessels in the liver, allantois, 

 spleen, and red bone-marrow, and in certain localities in the 

 connective tissue, by mitotic division of previously existing 

 nucleated corpuscles, in part to be formed endogenously within 

 special cells in the liver and perhaps other organs. Still later 

 the nucleated corpuscles give place in the blood of the mammal 

 to non-nucleated erythrocytes. Many of these are doubtless 

 derived from the nucleated corpuscles, but some appear to be 

 produced in the interior of certain cells of the connective tissue, 

 and are non-nucleated from the start. 



In the mammal in extra-uterine life the chief seat of forma- 

 tion of the red blood-corpuscles is the red marrow of the bones 

 of the skull and trunk, and of the ends of the long bones of the 



FIG. 4. CURVE SHOWING PROPOR- 

 TION 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 diges- 

 tion the number of lymphocytes in 

 the blood is greatly increased, both 

 absolutely and relatively to the 

 number of the other leucocytes. 



