1869.] Red Blood-corpuscle of Oviparous Vertehrata. 317 



often mixed with water or some other fluid, But it is possible to place 

 blood-cells under the microscope for examination so quickly, and with such 

 slight disturbance, that they may be satisfactorily examined before the 

 nuclei have begun to form. They may then be shown to be absolutely 

 structureless throughout ; and, moreover, as the examination is continued 

 the gradual formation of the nuclei can be traced. The chief points to be 

 attended to are — to mount a drop of blood as quickly as possible, to avoid 

 as much as possible any exposure to air, to avoid as much as practicable 

 contact of any foreign substance with the drop, or any disturbance of it. 



After many trials of various plans, I find that the following will often 

 succeed sufficiently well. Having the microscope, and everything else 

 which is required, conveniently arranged for immediate use, an assistant 

 secures the animal which is to furnish the blood (say, a frog or a newt), 

 in such a way that the operator may cleanly divide some superficial vessel, 

 as the femoral or humeral artery. He then instantly touches the drop of 

 blood which exudes with the under surface of the glass which is to be 

 used as the cover, immediately places this very lightly upon the slide, and 

 has the whole under the microscope with the least possible delay. Thus 

 for several seconds the blood-cells may be seen without any trace of nuclei ; 

 then, as the observation is continued, these gradually, but at first very faintly, 

 appear ; and the study of their formation affords strong proof of their 

 absence from the living cells. 



The "nucleus" first appears as an indistinct shadowy substance, usually, 

 but not always, about the centre of the cell. The outline of it can hardly, 

 for some seconds, be defined ; but it gradually grows more distinct. Often 

 some small portion of the edge appears clear before the rest. At the same 

 time the nucleus is seen to be paler than the surrounding substance. Syn- 

 chronously with this change — and this is noteworthy — the outline of the 

 corpuscle (the "cell- wall") becomes broader and darker. What was at 

 first a mere edge of homogeneous substance, becomes at length a dark 

 border sharply defined from the coloured matter within. Thus a corpuscle, 

 at first absolutely structureless, homogeneous throughout, is seen gradually 

 to be resolved into central substance or nucleus, external layer or cell- 

 wall, and an intermediate, coloured though very transparent, substance. 

 But — and this is significant — these changes are not always thus fully carried 

 out. It not seldom happens that the nucleus does not appear as a central 

 well-defined regularly oval mass. Sometimes it never forms so as to be 

 clearly traced in outline, but remains as an irregular shapeless mass, in its 

 greater portion very obscure. Sometimes only a small part, if any, of an 

 edge can be recognized, most of it appearing to blend indefinitely with the 

 rest of the cell-substance. Sometimes it happens that in many corpuscles 

 the formation of a nucleus does not proceed even so far as this. No 

 distinct separation of substance can anywhere be seen, but shadows, more 

 or less deep, here and there indicate that there is greater aggregation of 

 matter at some parts than at others. Occasionally some of the cells 



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