THE BLOOD. 



Development of the Blood-Corpuscles. 



The first formed blood-corpuscles of the human embryo differ much 

 in their general characters from those which belong to the later periods 

 of intra-uterine, and to all periods of extra-uterine life. Their manner 

 of origin is at first very simple. 



Surrounding the early embryo is a circular area, called the vascular 

 area, in which the first rudiments of the blood-vessels and blood-corpus- 

 cles are developed. Here the nucleated embryonal cells of the meso- 

 blast, from which the blood-vessels and corpuscles are to be formed, 

 send out processes in various directions, and these joining together, 

 form an irregular meshwork. The nuclei increase in number, and col- 

 lect chiefly in the larger masses of protoplasm, but partly also in the 



'0. 



, Fig- 141 - Part of the network of developing blood-vessels in the vascular area of a guinea-pie, 

 w, blood-corpuscles becoming free in an enlarged and hollowed-out part of the network ; a, process 



processes. It appears that haemoglobin then makes its appearance in cer- 

 tain of these nucleated embryonal cells, which thus become the earliest 

 red blood-corpuscles. The protoplasm of the cells and their branched 

 net-work in which these corpuscles lie then become hollowed out into a 

 system of canals inclosing fluid, in which the red nucleated corpuscles 

 float. The corpuscles at first are from about ^-g^o * ysW f an i nc h 

 (10/x to IQ/j) in diameter, mostly spherical, and with granular contents, 

 and a well-marked nucleus. Their nuclei, which are about -g-^Vo f an inch 

 (5/;-) in diameter, are central, circular, very little prominent on the sur- 

 faces of the corpuscles, and apparently slightly granular or tuberculated. 



The corpuscles then strongly resemble the colorless corpuscles of the 

 fully developed blood, but are colored. They are capable of amoeboid 

 movement and multiply by division. 



When, in the progress of embryonic development, the liver begins to 

 be formed, the multiplication of blood-cells in the whole mass of blood 

 ceases, and new blood-cells are produced by this organ, and also by the 



