64 THE BODY AT WORK 



The fluid inside it is the plasma of the blood. The nuclei and 

 their envelopes are blood-corpuscles. This, if it be a true 

 story, is a comprehensive way of making bloodvessels and 

 blood at the same time. Doubts have been thrown upon its 

 accuracy, but many leading histologists strenuously maintain 

 that this description is correct. 



At a certain period all nucleated red corpuscles disappear 

 from mammalian blood. Non-nucleated corpuscles take their 

 place. How are the latter formed ? For a short stage of 

 embryonic life nucleated cells containing blood-pigment are 

 seen, or are supposed to be seen, in the liver there is, unfortu- 

 nately, great difficulty in distinguishing them with certainty 

 from young liver-cells ; later they are seen in the spleen ; 

 throughout the whole of life they are to be seen in the marrow 

 of bone. The nucleated cells give origin to the non-nucleated 

 corpuscles. It is hardly legitimate to call these cells persistent 

 embryonic corpuscles. Yet the chain which connects the cells 

 which in the embryo are capable of dividing into pairs of 

 nucleated red blood-corpuscles, and the cells which, assuming 

 the role of parent cells, do not accumulate haemoglobin for their 

 own purposes, but for the benefit of the red corpuscles which 

 split off from them, is probably unbroken. In this sense they 

 are persistent embryonic corpuscles which have deserted the 

 blood-stream, and have taken shelter in certain tissues which 

 are particularly favourable for cell division. The situations in 

 which they hide themselves are singularly suggestive. In the 

 liver there is an abundant supply of nutriment, more abun- 

 dant than in any other part of the body of the embryo. Later, 

 in the spleen, red blood - corpuscles are being destroyed. 

 Materials available for making new ones must therefore be set 

 free. The inside of a hollow bone is a peculiarly sheltered 

 situation. The fat cells of marrow accumulate there after a 

 time ; but within some bones the marrow develops very 

 little fat ; hence it shows the red colour, which is due to its 

 abundant bloodvessels. This " red marrow " is the most 

 important seat of the manufacture of red blood-corpuscles in 

 adult life. Unfortunately, when we try to answer the question, 

 How are they formed ? we are obliged to speak with caution. 

 Some histologists assert that the nucleated cells divide, and 

 that one of the two daughter cells accumulates haemoglobin, and 



