DEVELOPMENT OF BLOOD. 101 



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 differs also, 

 and it will be well perhaps to consider this first. 



In the process of development of the embryo, the plan, 

 so to speak, of the heart and chief blood-vessels is first 

 laid out in cells. Thus the heart is at first but a solid 

 mass of cells, resembling those which constitute all other 

 parts of the embryo ; and continuous with this are tracts 

 of similar cells the rudiments of the chief blood-vessels. 



The formation of the first blood corpuscles is very 

 simple. While the outermost of the embryonic cells, of 

 which the rudimentary heart and its attendant vessels are 

 composed, gradually develop into the muscular and other 

 tissues which form the walls of the heart and blood-vessels, 

 the inner cells simply separate from each other, and form 

 blood-cells ; some fluid plasma being at the same time 

 secreted. Thus, by the same process, blood is formed, and 

 the originally solid heart and blood-vessels are hollowed out. 



The blood-cells produced in this way, are from about 

 Won to TsW f an i nc h i* 1 diameter, mostly spherical, but 

 some of them oval, pellucid and colourless, with granular 

 contents, and a well-marked nucleus. Gradually, they 

 acquire a red colour, at the same time that the nucleus 

 becomes more defined, and the granular matter clears 

 away. Mr. Paget describes them as, at this period, cir- 

 cular, thickly disc -shaped, full-coloured, and, on an average, 

 about -g-^Vo of an inch in diameter ; their nuclei, which are 

 about 5 O r of an inch in diameter, are central, circular, 

 very little prominent on the surfaces of the cell, and appa- 

 rently slightly granular or tuberculated. 



Before the occurrence, however, of this change from 

 the colourless to the coloured state in many instances, 

 probably, during it, and in many afterwards, a process of 

 multiplication takes place by division of the nucleus and 

 subsequently of the cell, into two, and much more rarely, 



