206 
BLOOD-DISCS OP KEPTILES AND FISHES. 
there is considerable diversity as to the size of the discs ; but 
the largest particles are found in the group of Amphibia , and 
especially in those species which retain their gills through 
life. The oval discs of Frogs (fig. 117) have a long diameter 
of about 1-1000th of an inch, and a transverse diameter of about 
1-1800th. Those of the perennibranchiate Amphibia (§ 87) 
may even be distinguished by the naked eye; those of the 
Siren having a long diameter of about 1-435th of an inch, whilst 
in the Proteus (fig. 118) the long diameter is stated occasionally 
to reach 1-337th of an inch. In Fishes, also, the size of the 
blood-discs is variable; they are 
sometimes smaller (fig. 119), though 
generally larger, than those of the 
Frog; but they never approach those 
of the last-named remarkable ani¬ 
mals. Hence the great size of the 
Fig. 119 .—Blood Corpuscles of blood-discs of the Curious Lcpido- 
Roach * siren (fig. 41) is strongly indicative 
corptcie of the Reptilian affinities of that 
treated with water. species. 
231. It is by observing the large blood-discs of the Frog, 
and still better those of the Proteus and Siren, that we can 
obtain the best information as to their structure. They are 
evidently flattened cells , having an envelope or cell-wall, which 
consists of an extremely delicate membrane, and which con¬ 
tains a fluid. The nucleus consists of an assemblage of minute 
granules, which seem adherent to each other and to the wall 
of the cell; and it corresponds, in all essential particulars, to 
the nuclei of the cells of other Animal tissues (§ 32). The 
fluid contained in the cells has a red colour.; and it is to this 
that the peculiar hue of the blood of Yertebrata is owing. 
When we are looking at a single layer of blood-discs, how¬ 
ever, their red colour is not apparent, but they have rather 
a yellowish tint; and it is only when we look through a 
number at once, that the characteristic hue is seen. The 
fluid is of about the same density as that in which the par¬ 
ticles float; and thus neither will have a tendency to pass 
towards the other. But, if we dilute the liquor sanguinis 
with water, the fluid outside the cells will have a tendency 
to pass towards their interior, according to the law of Endos- 
mose. The cells will in consequence be first distended, and 
