70 Royal Society : — 



evoked such various and even contradictory opinions of its nature. 

 But without attempting here any history of these, it may be shortly 

 said that amongst the conclusions now, and for a long time past, 

 generally accepted, a chief one is that a fundamental distinction 

 exists between the red corpuscle of Mammalia and that of the other 

 vertebrate classes — that the red cell of the oviparous vertebrata 

 possesses a nucleus which is not to be found in the corpuscle of the 

 other class. This great distinction between the classes has of late 

 years been over and over again laid down in the strongest and most 

 unqualified terms. 



But I venture to ask for a still further examination of this im- 

 portant subject. 



As the oviparous red cell is commonly seen, there can be no doubt 

 whatever about the existence of a "nucleus" in its interior. It is too 

 striking an object to escape any eye; but I submit that its existence is 

 due to the circumstances under which the corpuscle is seen, and the 

 mode in which it is prepared for examination. I think it can be 

 shown that the so-called nucleus is the result of the changes which 

 the substance of the corpuscle undergoes after death (and which are 

 usually hastened and exaggerated by exposure), and the disturbance 

 to which it is subjected in being mounted for the microscope. When 

 a drop of blood is prepared for examination, little or no attention is 

 given to the few seconds, more or less, which are consumed in the 

 manipulation. It is usually either pressed or spread out on the glass 

 slip, and 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 satis- 

 factorily 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 con- 

 tact 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 suflftciently well. Having the microscope, and every- 

 thing 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, imme- 

 diately 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, api)car ; and the study of their formation affords strong 

 l>roof of their absence from the living cells. 



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

 \isually, but not always, about the centre of the cell. The outline of 

 it can hardly, for some seconds, be defined ; but it gradually grows 



