IO 6 NORMAL HISTOLOGY. 



structure, as well as nuclear fibrils. Pole-corpuscles and at- 

 traction-spheres have been described by Flemming as constant 

 constituents of the white blood-corpuscle. Division of these 

 elements in many instances undoubtedly is accompanied by the 

 regular cycle of karyokinesis ; very commonly, however, it is equally 

 certain, the colorless corpuscles are reproduced by direct, amitotic 

 division. Examination of actively moving cells under high amplifi- 

 cation emphasizes a distinction in the character of the protoplasm, 

 that part of the cell constituting its most advanced portion seem- 

 ing more homogeneous than the remainder of the body of the 

 cell. 



The colorless cells of human blood are larger than the red 

 corpuscles, but are much fewer in number, the ratio between the 

 two kinds of elements being, under normal conditions, about three 

 hundred and fifty red cells to one white corpuscle. The actual 

 number of white cells present, however, depends upon various cir- 

 cumstances, since during digestion the number of colorless elements 

 is increased, while fasting greatly reduces the proportion of the 

 leucocytes ; in general these cells are more numerous in venous than 

 in arterial blood. 



The colorless blood-cells must be regarded as playing a double 

 rdle : in addition to maintaining an ever-available store of reserve 

 active protoplasm with which to meet and to repair the destructive 

 processes taking place normally as well as in disease, they are 

 actively engaged in the absorption of solid and fatty matters, being 

 capable of taking up and carrying away injurious debris. Certain 

 of these cells the phagocytes of Metschnikoff seem especially 

 aggressive in their attacks against offending foreign substances, 

 within a limited degree including possibly the waging of a successful 

 warfare on obnoxious microbes. 



THE COLORED CELLS OF THE BLOOD. 



The adult mammalian red blood-cell represents a condition of 

 retrogression, since in its development it has suffered the loss of 

 its nucleus and a profound metamorphosis of its protoplasm, changes 

 of such importance that some authorities dispute the propriety of 

 regarding the mammalian red blood-corpuscles as true cells. The 

 presence or absence of the nucleus within the colored corpuscle, 

 together with its general form, furnishes a basis for a division of all 

 vertebrate bloods into 



A. Those having nucleated, oval red corpuscles : including 

 fishes (except cyclostomata, which have round, discoidal cells, as the 

 lamprey), amphibians, reptiles, and birds. 



E. Those having non-nucleated, round, discoidal red cor- 



