42 PRACTICAL HISTOLOGY. 



completely transparent. Hence we may infer that 

 the opacity of the unaltered blood is due to the 

 presence of the red particles. 

 Preparation 8. Examination of Frog's Blood. 



The simplest way to obtain frog's blood for exami- 

 nation is to cut off the tip of one of the digits, having 

 previously wiped it dry with a cloth, and to collect 

 upon a clean cover-glass the small drop of reddish 

 fluid which exudes. The glass is then inverted upon 

 a slide and the drop is examined. The blood so 

 obtained is mixed with lymph, and the corpuscles 

 are consequently less crowded and better adapted 

 for observation than when the blood is undiluted. 

 To procure blood unmixed with lymph the animal 

 should be pithed, and laid upon its back. The heart 

 is then exposed and snipped with scissors, and a small 

 drop of the blood which exudes is taken up with a 

 glass rod, transferred to a slide and covered. But 

 before placing the cover-glass down in the usual way 

 a small length (J inch) of a delicate hair should be 

 placed in the drop, so that when the cover-glass set- 

 tles down, the corpuscles, here comparatively large, 

 may not be crushed by its weight. 



When the preparation obtained from either of 

 these sources is examined, it will at once be seen 

 that the colored corpuscles are larger and fewer in 

 number than in human blood, and do not tend to 

 form rouleaux ; that they have an elliptical outline 

 when lying flat, but when seen edgeways look quite 

 narrow and pointed at the ends, with a slight and 

 gradual bulging at either side; so that, although 

 disk-shaped, like the mammalian blood-corpuscles, 

 so far from being biconcave, they are biconvex. The 

 bulging is due to the presence in the middle of the 

 corpuscle of another part besides the stroma and the 

 colored substance found in the mammalian disk. 

 This, the so-called nucleus, can readily be made out 

 in most of the corpuscles as a somewhat elongated, 

 colorless, and slightly granular elliptical body, about 

 a third the length of the corpuscle, and often lying 



