206 BLOOD-DISCS OF REPTILES 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 
Stren 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 Fisus, 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.—Brioop CorPuscLEs oF blood-dises of the curious Lepido- 
Roacz. siren (fig. 41) is strongly indicative 
a2, 704 corpuscles; « elvr of the Reptilian aflinities of that 
treated with water. species. 
231. It is by observing the large blood-dises 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 Vertebrata 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 sanguwinis 
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 
