72 Uni/al Sort'rf//. 



common newt tlic cell.«, wliicli are larger than those of tlie frog, re- 

 main, as I have noticed, for a longer period without any appearance 

 of nuclei. But even in the frog it can be satisfactorily demonstrated 

 that the corpuscle is structureless. 



I have found, too, that the observation succeeds best with the 

 blood of animals which are healthy and vigorous. Thus the first 

 observations upon fresh animals are usually the most satisfactory. 

 After they have been repeatedly wounded or have lost much blood, 

 the cells are more prone to undergo the changes which result in the 

 production of nuclei. 



Again, the formation of nuclei may be hastened, and their ap- 

 pearance rendered more distinct at last, by various reagents. Acids 

 and many other reagents are well known to have this effect. The 

 addition of a small quantity of water acts in the same way, but less 

 energetically. It hastens the appearance of an indistinct nucleus, 

 but interferes with the formation of a well-defined mass, so that, 

 after the addition of water, neither the outline of the cell nor of the 

 nucleus becomes so strongly marked as it often does without it. 

 Exposure to air also promotes their formation ; indeed, as a rule, the 

 nuclei form best vmder simple exposure. Any disturbance of the 

 drop, as by moving the point of a needle in it, certainly hastens the 

 change ; and perhaps it is influenced by temperature. 



Sometimes, when the drop of blood has been skilfully mounted, 

 the majority of cells will remain for a long while without any trace 

 of nucleus ; but, again, in almost every specimen, the nucleus in some 

 few of the cells, particularly in those nearest the edges, begins to 

 appear so rapidly that it is hardly possible to run over the whole 

 field without finding some cells with an equivocal appearance. 



It would follow, of course, from these observations that, if the 

 living blood were examined in the vessels, the corpuscle would show 

 no trace of any distinction of parts; and this is so. Indeed, in my 

 earlier observations*, before I had learnt to mount a drop of blood 

 for observation in a satisfactory manner, I examined, at some length, 

 blood in the vessels of the most transparent parts I could select ; 

 and several observations on the web and lung of the frog and else- 

 where were satisfactory. But still, when the cells were thus some- 

 what obscured by intervening membrane, one could not generally 

 feel sure that the observation was so clear and complete, but that 

 a faintly marked nucleus might escape detection. While, therefore, 

 the result of observations on blood- cells in the vessels fully accords 

 with the description I have given, I do not think that the demon- 

 stration of the fact, that while living they have no nucleus, can be 

 made so plain and unequivocal as when they are removed from the 

 vessels. 



The question naturally arises. Why, then, does not a nucleus form 

 in the mammalian corpuscle ? But while it is accepted that the great 

 majority of these corpuscles exhibit no nuclei after death, excellent 

 observers still afliirm their occasional existence ; and I am convinced 



* Miulc inaiiy years ago. Otlicr observers have been unable to detect a nu- 

 cleus in the living cells witliin tlic vessels. 



