380 THE BLOOD, BY ALEXANDER ROLLETT. 



and Fishes, a different structure when the disk stands on its 

 edge. The optical section of the long axis appears here also 

 slender, elongated, and rounded at the extremities. The long 

 sides, however, have a projection at their centre (fig. 68, fr). 

 This prominence corresponds with an area situated near the 

 centre of the disk, which, in comparison with the remaining 

 coloured mass of the corpuscle, appears whiter than the rest. 

 This is sometimes more or less circular as in the Bird, or ellip- 

 tical as in the Frog, Triton, and land Salamander ; it is often 

 quite smooth, but also frequently presents fine indications of 

 dark points or striae. 



This spot corresponds to a structure which possesses no ana- 

 logue in the fully developed blood corpuscles of Man and 

 Mammals, but behaves itself quite differently from the remain- 

 ing substance of the corpuscle, and shows at least as great an 

 amount of agreement with the structure termed the nucleus in 

 other animal cells, as do the nuclei of the different cells with 

 one another. In common with most histologists, we shall 

 designate this structure as the nucleus of the blood corpuscles. 



The fully developed elliptical corpuscles of the camel* and 

 Auchenia are as destitute of a nucleus as the circular corpuscles 

 of Man and other Mammals. 



It thus appears that we may divide the blood corpuscles of 

 animals into two classes, the nucleated and the non-nucleated. 

 It must, however, be mentioned at once, that nucleated blood 

 corpuscles occur at an early period of the development of the 

 blood both in Man and Mammals. 



SIZE OF THE RED BLOOD CORPUSCLES. There is a large 

 amount of literature bearing on the subject of the micrometric 

 investigation of the blood. 



The considerably differing results of the measurements that 

 have been recorded have, for the most part, only a relative 

 value. The micrometer employed has not, as a rule, been 

 reduced to a definite standard. Exact comparison with a 

 standard, it is well known, is no easy matter even for macro- 



* Donne, Cours de Microscopic, etc., Paris, 1843, p. 70 ; Comptes Rendus, 

 T. xiv., p. 367. 



