116 MANUAL OF HISTOLOGY. 



we shall presently see, have been partly washed passively out of the 

 glands of the latter, partly have migrated actively from the same. They 

 may come likewise from the tissue of the spleen and medulla of bones, car- 

 ried off from these organs by the stream of venous blood flowing from them. 



And touching their further significance, they have been for many years 

 regarded, and we may now say rightly so, as cells which are destined to 

 pass into red corpuscles, and thus to cover the loss of the latter according 

 to the rate of their decay. The colourless cells serve to replace the red 

 therefore. 



This conjecture has, moreover, been confirmed by a remarkable discovery 

 of Von Recklinghausen, that frog's blood, collected in a vessel and kept 

 from evaporation, while the air about it is renewed several times daily, 

 will show, in from eleven to twenty-one days, a transformation of the 

 colourless corpuscles into the characteristic red cells of that animal. How 

 many or how few of the uncoloured cells undergo this change in the living 

 body is a question, however, which cannot at present be met by scientific 

 facts. The statements made on this point must necessarily vary greatly, 

 depending as they do on the hypothesis as to the amount of chyle and 

 lymph streaming daily into the blood, as well as on the still completely 

 unknown length of existence of the coloured blood-cells. It seems highly 

 probable, however, that a large number of these colourless elements never 

 attain this state, and pass to decay without being transformed into red cells. 



But we are also still in the dark as to how this change exactly takes 

 place. We only know so much that the white corpuscle is transformed 

 (usually diminishing in bulk) into a circular flat plate, and generates 

 within itself a yellow material at the same time that it loses its nucleus 

 and protoplasm. Among the groups of vertebrates in whose coloured 

 cells a nucleus occurs, the latter structure is permanent. 



Nor are we better enlightened as to the region in which this change 

 takes place. In some cases it appears to be over the whole circulatory 

 tract ; for we may remark in the blood of the three lower classes of ver- 

 tebrates, rare intermediate forms that is, besides the usual nucleated, 

 red corpuscles others of a much paler colour, with round or oval figure 

 (pale blood-corpuscles). These may be readily recognised, especially in 

 the large-celled blood of the frog and salamander (2). Then, again, very 

 similar cells may be found in the blood of the human and mammalian 

 spleen, of which it is difficult to say whether they still belong to the 

 lymph-corpuscles, or are already red blood-cells. Finally, similar inter- 

 mediate cells are met with, according to Bizzozero and Neumann, in the 

 medulla of bones, especially in the red species of medulla. 



REMARKS. 1. See Von RecTclinghausen in the Archiv. fur mikrosk. Anatomic, Bd. 

 ii. 137. 2. Comp. Wharton Janes' work. 



72. 



Though, from an anatomical point of view, blood may appear a toler- 

 ably simple tissue, with fluid intercellular substance, physiologically it is 

 a fluid of very complex constitution. In it we have the very focus of 

 vegetative activity, the stream upon which all the traffic of the system 

 takes place as it were. In it we must expect to find matters which serve 

 as well for the formation of tissues as for nutrition. These, however, may 

 still exist in several varieties of combination not met with in the tissues. 

 The most diverse products of metamorphosis also pass through the blood 

 on their way to excretion. We can hardly be surprised, then, that almost 



