218 ORIGIN OP THE RED BLOOD-DISKS. 



to move within the vessels of the vascular area. And when the action of the heart 

 commences, the blood is driven also through vessels which are formed, probably in a 

 manner similar to that above described, in the mesoblast of the body of the embryo. 



These first formed red blood-corpuscles are nucleated cells resembling the pale 

 corpuscles except in their colour and in the clearness of their protoplasm, and, like 

 the white corpuscles, they are capable of amoeboid movement, and of undergoing 

 -multiplication by division. It is uncertain whether, as stated by Kolliker and others, 

 any of the primary red blood-corpuscles are produced by direct transformation of 

 individual cells of the mesoblast, but (whether by accession of some of these last, by 

 division, or by a continuance of the original mode of formation), the numbers in- 

 crease considerably, and they are soon accompanied by colourless corpuscles. These 

 appear to be formed in great number in the embryonic liver as soon as this is de- 

 veloped, as well as in the lymphatic glands, spleen and thymus gland. It has been 

 supposed that the colourless corpuscles formed in these organs acquire colour, and 

 are converted into nucleated red corpuscles, but there is no direct evidence in favour 

 of this view. 



The primary nucleated red corpuscles are at length succeeded by smaller disk- 

 shaped red corpuscles without nuclei, having all the characters of the blood-disks of 

 the adult. This substitution proceeds gradually, until, long before the end of intra- 

 uterine life, the nucleated red corpuscles have almost entirely vanished from the blood. 

 According to Neumann, some are still to be met with even in the new-born child. 

 It is probable that they are converted into non-nucleated disks, but it is not known 

 how the transformation occurs. Probably the process is the same as that which 

 takes place in the case of the nucleated red corpuscles of the red marrow of the adult, 

 which are indeed in all likelihood the direct descendants of the embryonic nucleated 

 blood-corpuscles. 



Origin of the red blood-disks. 1. Intracellnlar origin. The disk-shaped 

 red corpuscles are produced in the interior of angioblastic connective tissue cells 

 in the following manner : 



Fig. 255. DEVELOPMENT OP RED CORPUSCLES IN CONNECTIVE TISSUE CELLS (ANGIOBLASTS). FROM THE 



SUBCUTANEOUS TISSUE OF THE NEW-BORN RAT. (E. A. S. ) 



h, a cell containing haemoglobin in a diffused form in the protoplasm ; h', one containing coloured 

 globules of varying size, and vacuoles ; h", a cell filled with coloured globules of nearly uniform size ; 

 /, /', developing fat cells. 



A part of the protoplasm of the cell acquires a reddish tinge (fig. 255, h}, and 

 after a time the coloured substance becomes condensed in the form of globules (h') 

 within the cells, varying in size from a minute speck to a spheroid of the diameter 

 of a blood-corpuscle, or even larger ; but gradually the size becomes more uniform 

 (fig. 255, h"). Some parts of the embryonic connective tissue, especially where a 

 vascular tissue, such as the fat, is about to be developed, are completely studded 

 with cells like these, occupied by a number of coloured spheroids and forming nests 

 of blood-corpuscles or minute "blood-islands." After a time the cells become 

 elongated and pointed at their ends, and processes grow out to join prolongations of 



