Cells and Tissues. 5 



there is no protrusion of true pseudopodia. Oil-globules and like 

 matters may be differentiated in the protoplasm : the surface of the 

 body may be covered with fine, hair-like protoplasmic structures, 

 called cilia, which exhibit constant waving movements, etc. (For 

 details, see special part — "Protozoa," p. 85). 



All other animals, the Metazoa, begin life as ova, small 

 corpuscles of living protoplasm provided with a nucleus, and at this 

 stage agree, in all essential respects, with the Amoeba. Unlike 

 Protozoa, however, they do not remain in this state : the ovum or egg 

 divides into a number of segments, each with its own nucleus, but, 

 instead of separating from one another, they remain in connection ; 

 such segments are called cells; the essential properties of the ovum, 

 as also of the Amoeba, are possessed by each one, and the body of the 

 Metazoon attains its definite form by repeated division and differen- 

 tiation of the cells. The perfect Metazoon, then, is an intimate 

 association of cells like an Amoeba, but showing greater or smaller 

 modifications. 



Cells consist of protoplasm, like that of the Amoeba, either 

 throughout their existence, or only in the young stages. There is, 

 in the protoplasm, a vesicular nucleus con- 

 taining a watery liquid (a chromatin), and a 

 network of delicate threads (the chromatin). 

 One or more rounded nucleoli are also frequently 

 present, often appearing as thickened spots in fe'r^-^VC^^? 

 the network. The cells are capable of di- '\ilWz_>^'?l"' 

 viding; division is preceded by peculiar 'KvV/^i'J^&f'^^' 

 changes in the network, then the nucleus Vs^*-^I$isi"*5 

 divides, and finally the protoplasm separates 

 into two pieces. The cell exhibits all the other Fig. 2. A cell, n 

 essential properties of the Amoeba; it absorbs rnSar"'thread°^'''' 

 oxygen, takes in food, and so on. protoplasm.— Orig.' 



It has recently been shown for many cells that the 

 protoplasm consists of a homogeneous matrix, and of numerous fine threads 

 besides the granules just mentioned. Sometimes many nuclei are present 

 in a cell, which is then considered to be an incompletely divided cell-mass 

 the nucleus has divided, but no division of the cell body has resulted. 



The division of the nucleus is, in a few cases, direct; it simply 

 constricts in one place, and then separates into two pieces. As a rule, however 

 nuclear division is indirect, and then it takes place in a complicated way, 

 as follows : First, the whole nuclear network forms a long coiled thread 

 (Fig. 3, 1), which breaks up into a number of rod-like or curved pieces, 

 the chromosomes; next, the nuclear membrane disappears (Fig. 3,2), and 

 each chromosome splits lengthways in two (Fig. 3, 3) ; but before this, two 

 little bodies, the centrosomes, from which fine threads radiate to the chi-omo- 

 somes, may be noticed in the protoplasm.* Then these threads shorten. 



* One or two centrosomes may be noticed lying near the nucleus in many resting 

 cells (i.e., cells which are not ready for division). Possibly they are as constant a 

 constituent of the cell as is the nucleus. 



