THE BLOOD. 



155 



Pig, 



only colorless corpuscles, but all vertebrates have colored cells 

 which invariably outnumber the other variety, and display 

 forms and sizes 

 which are sufficient- 

 ly constant to be 

 characteristic. In all 

 groups below mam- 

 mals the colored cor- 

 puscles are oval, 

 mostly biconvex, 

 and nucleated dur- 

 ing all periods of the 

 animal's existence ; 

 in mammals they are 

 circular biconcave 

 disks (except in the 

 camfil tribe, the cor- 

 puscles of which are 

 oval), and in post- 

 embryonic life with- 

 out a nucleus ; nor 

 do they possess a 

 cell-wall. The red 



cells vary in size in different groups and sub-groups of animals, 

 being smaller the higher the place the animal occupies, as a 



general rule ; thus, they 

 are very large in verte- 

 brates below mammals, 

 in some cases being al- 

 most visible to the un- 

 aided eye, while in the 

 whole class of mam- 

 mals they are very mi- 

 nute ; their numbers 

 also in this group are 

 vastly greater than in 

 others lower iu the 

 scale. 



The average size in 

 man is ^^^^ inch ('0077 

 mifi.) and the nnmber 



Fig. 140.— Photograph of colored corpnscles of frog. . ,. .,_. , 



1 X 370. (After Flint.). in a cubic millimetre 



Leucocytes of human blood, showing amoe- 

 boid movements (Landois). These movements are 

 not normally in the blood-vessels so marked as pic- 

 tured here, so that the figure represents an extreme 

 case. 



