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 dia- 

 meter of the human disks, while their length is about 

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



a Man. 



d 



BLOOD-DISKS. 



b Blennv. c Frog. 



,/ 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 



