90 DEVELOPMENT OF BLOOD. 



the oxygen. The proportion of nitrogen is in both very 

 small. 



It is most probable that the carbonic acid of the blood 

 is partly in a state of simple solution, and partly in a state 

 of weak chemical combination. The portion of the car- 

 bonic acid which is chemically combined, is contained 

 partly in a bicarbonate of soda, and partly is united with 

 phosphate of the same base. The oxygen is combined 

 chemically with the heemoglobin of the red corpuscles 

 (pp. 75 and 85). 



That the oxygen is absorbed chiefly by the red corpuscles 

 is proved by the fact that while blood is capable of 

 absorbing oxygen in considerable quantity, the serum 

 alone has little or no more power of absorbing this gas 

 than pure water. 



Development of the Blood. 



In the development of the blood little more can be traced 

 than the processes by which the corpuscles are formed. 



The first formed blood-cells of the human embryo differ 

 much in their general characters from those which belong 

 to the latter 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 embr3 r o ; 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 



