28 



EVENINGS AT THE MICROSCOPE. 



size of these corpuscles in various animals, and such tables 

 are. very useful; but we must bear in mind that the 

 average dimensions only are to be looked for ; since in 

 any given quantity of blood under examination, we shall 

 not fail to see that some disks exceed, while others come 

 short of, the dimensions of the majority. 



Generally speaking, the blood-disks in Birds and in 

 Fishes are about equal in size: their form is, however, 

 that of a more elongated ellipse in Birds than in Fishes. 

 They may be set down as averaging in breadth the diameter 

 of the human disks, while their length is about half as 

 much again, or a little more, in most Birds. 



BLOOD DISKS. 



& Blenny. c Frog. 



d Newt. 



It is in Reptiles that we meet with the largest disks, 

 and especially in those naked-skinned species, the Frogs 

 and Newts. A large species inhabiting the American 

 lakes — Siren lacertina — has disks of the extraordinary 

 size of l-400th of an inch long by l-800th broad, or about 

 eight times as large as those of Man, in linear measure. 

 Our common Newts afford us the largest examples among 

 British animals, but they do not reach above half the size 

 just mentioned. 



Taking this drop of blood from my finger as a standard 

 of comparison, we find, on applying the micrometer, that 



