THE FLUIDS OF THE BODY 63 



wishes that he could tell it with assurance ; but, unfortunately, 

 there are many uncertainties, due to conflicting testimony, in its 

 earlier chapters. It is unlikely that a blood-corpuscle lives for 

 long. A month or six weeks is probably the term of its exist- 

 ence. The rapidity with which the stock is replenished after 

 bleeding shows that there must be ample provision in the body 

 for making blood-corpuscles. The rate at which they dis- 

 appear after they have been added in excess shows that there 

 is an equally effective mechanism for destroying them. If half 

 as many again as the animal already possesses be injected into 

 its veins, the number is reduced to its normal limit in about 

 ten days. It is clear that they can be made and can be de- 

 stroyed with great facility, and it seems a legitimate inference 

 that production and destruction are constantly taking place. 

 Regarding the way in which they are destroyed there is no 

 uncertainty. We shall refer to this subject when describing the 

 functions of the spleen. But how are they made ? We can 

 sketch their history in outline, but the evidence is conflicting 

 with regard to all matters of detail. 



In early stages of embryonic life all red blood-corpuscles 

 are nucleated, as they are permanently in birds and the other 

 classes of vertebrates below mammals. In embryonic mammals 

 they multiply by division whilst circulating in the blood, just as 

 in the newt. But it is generally believed that this is not the 

 most important source of new ones. During the earliest stages 

 of growth they are being formed in enormous numbers. Such 

 instances of division as can be seen in circulating blood appear 

 to be all too infrequent to account for their rapid multiplica- 

 tion, and there can be no doubt but that a more complicated 

 method of production is more important. Their formation is 

 described as taking place " endogenously." Certain cells 

 termed " vaso-formative," or " vaso-sanguiformative," reach a 

 considerable size, and become stellate in form, or branched. 

 Their nuclei divide without the cell dividing. Each nucleus 

 accumulates a little haemoglobin round it. A space filled 

 with fluid appears inside the cell. The nuclei project into this 

 space. Then they drop off with their envelopes of haemo- 

 globin. The outer shell of the big vaso-formative cell becomes 

 the wall of a capillary bloodvessel. By its branches it links 

 up with other vaso-formative cells, making a network of vessels. 



