THE NUTRITIVE FLUIDS. 1077 



of cells during karyokinesis (mitosis) ; it is based chiefly on the work of Flem- 

 raing, Strasburger, E. van Beneden, Rabl, O'Hertwig, Henneguy, etc. A. Resting 

 nucleus. Nucleolus and nuclear membrane visible. A centrosome is represented 

 near the nucleus. B and C. Skein or spirem. Chromatic filaments much convo- 

 luted. Evidence of longitudinal splitting begins to be distinct in several parts. 

 The centrosome has divided ; the nuclear membrane is becoming indistinct. The 

 two centrosomes are widely separated, and the space between them is occupied by 

 the achromatic spindle. (Two arrows indicate the position which the centrosomes 

 will ultimately occupy ; during their passage to these points the achromatic spindle 

 seems to be within the nucleus.) The nuclear membrane has disappeared. D. 

 Mother star, monaster. The nuclear segments (chromosomes) resulting from the 

 breaking-up of the chromatic filament into fragments of nearly equal length have 

 moved toward the equator, of the spindle, where they now form an equatorial plate. 

 These segments are all split longitudinally. E. MetapJiase. One half of each 

 chromosome moves toward one pole and the other half toward the other 

 pole, being guided toward the centrosomes by the achromatic filaments. F. 

 Daughter stars or diaster. G. Daughter skeins or dispirem, beginning to form. 

 Segments in the form of thick loops not closely packed. H. Daughter skeins or 

 dispirem, formed. Segments more closely packed and less distinct, owing to the 

 formation of anastomoses. 1. Resting daughter nuclei. Cell completely divided 

 into two, but bridges remain between them in the region previously occupied by 

 the achromatic filaments, these being specially distinct in certain cells, e. g. y 

 prickle-cells. The nucleus has a distinct nuclear membrane and a nucleolus. 



In the reproduction of cells by direct division the process is brought about 

 either by segmentation or by gemmation. In reproduction by segmentation or 

 fission the nucleus becomes constricted in its centre, assuming an hour-glass shape, 

 and then divides into two. This leads to a cleavage or division of the whole 

 protoplasmic mass of the cell ; and thus two daughter cells are formed, each con- 

 taining a nucleus. These daughter cells are at first smaller than the original 

 mother cell ; but they grow, and the process may be repeated in them, so that 

 multiplication may take place rapidly. In reproduction by gemmation a budding- 

 off or separation of a portion of the nucleus and parent-cell takes place, and, 

 becoming separated, forms a new organism. 



The cell-wall, which is not an essential constituent, and in fact is often absent, 

 is merely the external layer of the protoplasm, firmer than the rest of the cell, 

 and often thickened by the deposit in it of certain chemical substances. It forms 

 a flexible, transparent, finely striated membrane, sometimes furnished with minute 

 pores, so as to be permeable to fluids. 



THE NUTRITIVE FLUIDS. 



The circulating fluids of the body, which subserve its nutrition, are the blood, 

 the lymph, and the chyle. 



THE BLOOD. 



The blood is an opaque, rather viscid fluid, of a bright-red or scarlet color 

 when it flows from the arteries, of a dark-red or purple color when it flows from 

 the veins. It is salt to the taste, and has a peculiar faint odor and an alkaline 

 reaction. Its specific gravity is about 1.060, and its temperature is generally 

 about 100 F., though varying slightly in different parts of the body. 



General Composition of the Blood. Blood consists of a faintly yellow fluid, 

 the plasma or liquor sanguinis, in which are suspended numerous minute particles, 

 the blood-corpuscles, the majority of which are colored and give to the blood its 

 red tint. If a drop of blood is placed in a thin layer on a glass slide and examined 

 under the microscope, a number of these corpuscles will be seen immersed in 

 the clear fluid plasma. 



The Blood-corpuscles are chiefly of two kinds: (1) colored corpuscles or 



