MICROSCOPIC OBJECTS. 33 



will be vain to hope for the growth of sound potatoes, 

 for some years, where diseased ones have previously- 

 existed, unless they be sown entire — which method, by 

 planting in drills, would be not less productive — and 

 thus the disease be prevented. 



Blood-discs. — ^The red corpuscles in the blood of man 

 are always of a circular form, and in their normal state 

 are flattened discs without nuclei. If a drop of water be 

 added to the blood, they become spherical by endosmose. 

 In all air-breathing oviparous vertebrated animals, the 

 blood- corpuscles are oval, and a nucleus may be observed 

 within each of them. This nucleus is rendered very 

 distinct by the addition of a drop of diluted acetic acid. 

 — ^The superior size of the blood- discs in the Siren 

 lacertina has enabled Professor Owen to make a minute 

 examination of the contained nucleus. He has distinctly 

 observed that the nucleus consists of a cluster of from 

 20 to 30 bright spherical nucleoli inclosed in a trans- 

 parent capsule, in the centre of the oval- shaped flattened 

 blood- disc. The length of the disc is 4-J^(jth of an inch ; 

 while the average diameter of that of human blood is 

 -g-J^Qth of an inch. The magnifying power of a micro- 

 scope for the examination of these matters should range 

 from 400 to 800 diameters. For preparing and measur- 

 ing Blood- discs see * Micrographia.' 



Bone. — For microscopic examination, bone should be 

 cut into thin sections about ^-^th of an inch in thickness, 

 and mounted on glass slides. When animal tissues are 

 consohdated by the deposition of earthy matter within 



