VEKTEBRATED ANIMALS. 271 



granules which may have been previously seen in active molecular move- 

 ment within the corpuscle. When the white corpuscles in a drop of 

 freshly drawn blood are carefully watched for a short time, they may be 

 observed to undergo changes of form, and even to move from place to 

 place, after the manner of Amoeba ( 403). When thus moving, they 

 engulf particles which lie in their course such as granules of vermilion 

 that have been injected into the blood-vessels of the living animal, and 

 afterwards eject these, in the like fashion. Such movements will continue 

 for some time in the colorless corpuscles of cold-blooded animals, but 

 still longer if they are kept in a temperature of about 75. The move- 

 ment will speedily come to an end, however, in the white corpuscles of 

 Man or other warm-blooded animals, unless the slide is kept on a warm 

 stage at the temperature of about 100 F. A remarkable example of an 

 extreme change of form in a white corpuscle of Human blood, is repre- 

 sented in Fig. 458. Similar changes have been observed also in the cor- 

 puscles floating in the circulating fluid of the higher Invertebrata, as the 

 Crab, which resemble the ' white' corpuscles of Vertebrated blood, 

 rather than its 'red' corpuscles, 

 these last, in fact, being altogether FIO. 458. 



peculiar to the circulating fluid of 

 Vertebrated animals. 



667. In examining the Blood mi- 

 croscopically, it is, of course, impor- 

 tant to obtain as thin a stratum of 

 it as possible, so that the corpuscles 

 may not overlie one another. This 

 is best accomplished by selecting a 

 piece of thin glass of perfect flatness, 

 and then, having received a small 

 drop of Blood upon a glass slide, 

 to lay the thin-glass cover not upon 

 this, but with its edge just touching 



the edge of the drop; for the blood Altered ^.^ c les of Blood< an 



Will then be drawn-Ill by Capillary hour after having been drawn from the finger. 



attraction, so as to spread in a uni- 

 formly-thin layer between the two glasses. Such thin films may be pre- 

 served in the liquid state by applying a cover-glass and cementing it with 

 gold size before evaporation has taken place; but it is preferable first to 

 expose the drop to the vapor of Osmic acid, and then to apply a drop of 

 a weak solution of Acetate of Potass; after which a cover-glass may be 

 put on, and secured with gold-size in the usual way. It is far simpler, 

 however, to allow such films to dry without any cover, and then merely 

 to cover them for protection; and in this condition the general charac- 

 ters of the corpuscles can be very well made-out, notwithstanding that 

 they have in some degree shrivelled by the desiccation they have under- 

 gone. And this method is particularly serviceable, as affording a fail- 

 means of comparison, when the assistance of the Microscopist is sought 

 in determining, for Medico-legal purposes, the source of suspicious 

 blood-stains; the average dimensions of the dried blood-corpuscles of the 

 several domestic animals being sufficiently different from each other, and 

 from those of Man, to allow the nature of any specimen to be pro- 

 nounced-upon with a high degree of probability. 



668. Simple Fibrous Tissues. A very beautiful example of a tissue 

 of this kind is furnished by the membrane of the common Fowl's egg; 



