1038 VERTEBRATE!) 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 represented in fig. 769. 

 Similar changes have been observed also in the corpuscles floating in 

 the circulating fluid of the higher invertebrate, as the crab, which 

 resemble the ' white ' corpuscles of vertebrated blood, rather than 

 its ' red ' corpuscles these last, in fact, being altogether peculiar to 

 the circulating fluid of vertebrated animals. 



In examining the blood microscopically it is, of course, impor- 

 tant to obtain as thin a stratum of it as possible, so that the cor- 

 puscles may not overlie one another. 

 This is best accomplished by selecting 

 a pi ece f *hi n gl ass of perfect flat- 

 ness, and then, having received a 

 small drop of blood upon a glass 

 s lid e > to lay the thin glass cover not 

 upon this, but with its edge just 

 touching the edge of the drop : for 

 the blood will then be drawn in by 

 FIG. no.-a, blood-cell of a frog in capillary attraction, so as to spread 

 the act 'of engulfing a rod of in a uniformly thin layer between 

 Bacillus anthracis, observed in the the two glasses. Such thin films 



S'ttS^fel !S -y be preserved in the liquid state 

 later. (After Metschmkoff ; highly by applying a cover glass and ce- 

 magm'fied.) menting it with gold-size before 



evaporation has taken place ; but it 



is preferable first to expose the drop to the vapour 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 

 characters of the corpuscles can be very well made out, notwith- 

 standing that they have in some degree been shrivelled by the 

 desiccation they have undergone. This method is particularly ser- 

 viceable as affording a fair means of comparison, when the assist- 

 ance of the microscopist is sought in determining, for medico-legal 

 purposes, the source of suspicious blood-stains, the average dimen- 

 sions of the dried blood-corpuscle 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 pronounced upon with a 

 high degree of probability. 1 



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

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

 which (as may be seen by examining an egg whose shell remains 

 soft for want of consolidation by calcareous particles) consists of two 

 principal layers, one serving as a basis of the shell itself, and the 

 other forming that lining to it which is known as the membrana 



1 This is a matter which has given, rise to much discussion among experts. See 

 Proc. Amer. Micr. Soc. xiv. (1893), pp. 91-120 



