DEVELOPMENT OF BLOOD. 81 



of their action, too slight though it may be, at any given mo- 

 ment, for analysis by means now at our disposal. 



On the Gases contained in the Blood. 



The gases contained in the blood are carbonic acid, oxygen, 

 and nitrogen, 100 volumes of blood containing from 40 to 50 

 volumes of these gases collectively. 



Arterial blood contains relatively more oxygen and less 

 carbonic acid than venous. But the absolute quantity of car- 

 bonic acid is in both kinds of blood greater than that of 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. That portion of the carbonic 

 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 

 haemoglobin of the red corpuscles (pp. 69 and 77). 



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 bloodvessels 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 em- 

 bryo ; and continuous with this are tracts of similar cells 

 the rudiments of the chief bloodvessels. 



The formation of the first blood-corpuscles is very simple. 

 While the outermost of the embryonic cells, of which the ru- 

 dimentary heart and its attendant vessels are composed, gradu- 

 ally develop into the muscular and other tissues which form 

 the walls of the heart and bloodvessels, 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 



