238 Blood-corpuscle of Oviparous Vertebraia. [SSll aSuTK 
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 indis- 
tinct nucleus, but interferes with the formation of a well-defined 
mass, so that, after the addition of water, the outline neither 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 under 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 
elsewhere were satisfactory. But still, when the cells are thus 
somewhat 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 demonstration of the fact that, while hving, 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 affirm their occasional existence ; and I am 
convinced that an indistinct, imperfectly formed " nucleus " is often 
seen ; and the shadowy substance seen in many of the smaller ovi- 
parous cells after they have been mounted for some time, is very like 
that seen under similar circumstances in some of the corpuscles of 
Mammalia. Many, too, affirm that these corpuscles do not exhibit 
that distinction of wall and contents which is generally described. 
It appears to me that this difference of opinion depends on the 
* Made many years ago. Other observers liave been unable to detect a nucleus 
in the living cells witliin the vessels. 
