ITS DIFFERENT PHASES OF DEVELOPMENT IN THE ANIMAL SERIES. 
97 
Blood-corpuscles of the Mussel. 
71. The way in which I obtained the blood of the Mussel was this: — I first re- 
moved one valve of the shell with as little injury to the animal as possible, and 
allowed the contained fluid to drain away ; I then opened the great vessel proceeding 
from the heart to the anterior part of the body, and introducing the microscopical 
forceps towards the heart, took up enough of the blood for one examination. Not- 
withstanding this careful procedure, the blood was often mixed with foreign corpus- 
cles, such as the spermatozoa and ova of the animal. 
72. Kinds of Blood-corpuscles . — There were in the first place granule-cells, which, 
like those of the blood of the Whelk, were found by the time the blood could be 
examined, already for the most part agglomerated together, and presenting their 
cell-wall shot out into processes (figs. 1 and 2.). The granulous contents appeared 
as if fused, forming a clear, more or less strongly refracting mass. 
73. By the action of water this mass was broken up into separate granules, and at 
the same time the cell was observed to become gradually distended and rendered 
uniformly circular by the undoing of the processes into which its wall had shot. 
This undoing of the processes I have often, as in the case of the blood-cells of other 
animals, watched step by step. 
74. When the cell had become fully distended, the granulous contents were 
usually next seen to escape by the bursting of the cell-wall, and to accumulate around 
its outside, whilst a nucleus from about T w oth to - nAn jth of an inch in diameter, and 
now for the first time seen, remained in the interior ; the cell-wall, different from that 
of the granule-cell of the Whelk, continuing quite visible and otherwise entire (fig. 3.). 
75. Such was the condition in which the granule-cells presented themselves in the 
blood of fresh mussels. In the blood of mussels which had been kept for some days 
in the house without food or change of water, but still alive though weak, the granule- 
cells were found, for the most part, to continue uniformly distended after the blood 
was drawn, their cell-wall not shooting out into processes (figs. 4 and 5.). The con- 
tents appeared at the same time more broken up into granules. In this state the 
size of the cells was from 1 ^op th to °f an inch in diameter. They appeared 
in both coarsely and finely granular stages. 
76. Cells in which the nucleus is already distinctly visible without the application 
of any reagent, were few in number in the blood of perfectly fresh mussels, but in 
the blood of mussels which had been kept some days, they presented themselves in 
considerable numbers (figs. 9, 10, 11 and 12.). 
77. The corpuscles in which a nucleus had thus become distinctly visible were, as 
they occurred in the blood of fresh mussels, distinguished by the following charac- 
ters : — Circular but sometimes ovoid, somewhat prominent in the middle, with a 
uniform and strongly marked contour, of an opaline aspect, and measuring on an 
average about a^o th of an inch in diameter, many being both smaller and larger 
(figs. 6 and “J .). 
