Apri?riff ] BIood-corj)uscle of Ovi;parous Vertehrata, 237 
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 present throughout a granu- 
lar aspect. I have almost invariably observed, too, a relation between 
the distinctness of the nucleus and of the cell-wall. When the 
nucleus is well defined, the cell- wall is strongly marked ; when one 
is confused, the other is usually fainter. This, however, does not 
apply to colour ; on the contrary, when the nucleus is least coloured 
it contrasts more strongly with the surrounding cell. As a rule, 
the wall of the cell is more strongly marked than the nucleus. 
It vdll of course be said that the nuclei are present all the 
while, but are at first concealed by the surrounding substance — the 
contents of the cell. Thus the fact has been accounted for, that 
the nuclei are not so obvious at first as they subsequently become. 
But I think a careful comparison of cells will show that those in 
which a nucleus may be traced are not more transparent than 
others which are structureless ; and, moreover, when one cell over- 
laps another, the lower one is seen through the upper clearly enough 
to show that the substance of these cells is sufficiently transparent 
to allow of a nucleus being discerned if it exist. When a nucleus 
is fully formed, it hides that portion of the outline of a cell which 
lies beneath it. How is it, then, if the nucleus is present from the 
first, that the portion of the cell over which it subsequently appears 
is, for awhile, plainly seen ? 
The success of the observation is of course influenced by nume- 
rous circumstances. The rate at which the nuclei form in the 
corpuscles varies in different animals. I have usually found that 
in the common frog they are more prone to form than in many 
other animals — quicker than in most fishes, or even than in some 
birds. But this does not seem always to depend upon their larger 
size ; for in the common newt, the cells, which are larger than those 
of the frog, remain, as I have noticed, for a longer period without 
any appearance of nuclei. But even in the frog it can be satis- 
factorily 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 
appearance rendered more distinct at last, by various reagents. 
s 2 
