THE ANIMAL CELL. 1075 



Recent investigations tend to show that most living cells contain, in addition 

 to their protoplasm and nucleus, a minute particle which, on account of the power 

 it appears to possess of attracting the surrounding protoplasmic granules, is 

 termed the attraction-particle or eentrosome ; it usually lies near the nucleus. 

 The spherical arrangement of fibrillar rows of granules surrounding the central 

 particle is termed the attraction-sphere or centrosphere. These spheres are 

 usually double, and are connected by a spindle-shaped system of delicate fibrils 

 (achromatic spindle). They are best seen in young cells which are about to undergo 

 the process of division, a process believed to commence in these bodies. 



The process of reproduction of cells is usually described as being brought 

 about by indirect or by direct division. Indirect division or karyokinesis (karyo- 

 mitosis) has been observed in all the tissues generative cells, epithelial tissue, 

 connective tissue, muscular tissue, and nerve-tissue and probably it will ulti- 

 mately be shown that the division of cells always takes place in this way, and 

 that the process of reproduction of cells by direct division is, as is believed by 

 some observers, merely a sort of imperfect or abnormal karyokinesis. 



The process of indirect cell-division is characterized by a series of complex 

 changes in the nucleus, leading to its subdivision ; this being followed by cleavage 

 of the cell-protoplasm. Starting with the nucleus in the quiescent or resting 

 stage, these changes may be briefly grouped under the four following phases : 



1. Prophase. The nuclear network of chr omatin-filaments assumes the form of 

 a twisted skein or spirem, while the nuclear membrane and nucleolus disappear. 

 The convoluted skein of chromatin divides into a definite number of V-shaped 

 loops or chromosomes. Coincident with or preceding these changes the eentro- 

 some, or attraction-particle, which usually lies by the side of the nucleus, 

 undergoes subdivision, and the two resulting centrosomes, each surrounded by a 

 centrosphere, are seen to be connected by a spindle of delicate achromatic fibres, 

 the achromatic spindle. These centrosomes move away from each other one 

 toward each extremity of the nucleus and the fibrils of the achromatic spindle 

 are correspondingly lengthened. The centrosomes are now situated one at 

 either extremity or pole of the elongated spindle, and each is surrounded by a 

 centrosphere, from which fibrils radiate into the investing protoplasm. A line 

 encircling the spindle midway between its poles is named the equator, and 

 around this the V-shaped chromosomes arrange themselves in the form of a star, 

 thus constituting the mother star or monaster. 



2. Metaphase. Each V-shaped chromosome now undergoes longitudinal 

 cleavage into two equal halves or daughter chromosomes, the cleavage commenc- 

 ing at the apex of the V and extending along its divergent limbs. The daughter 

 chromosomes, thus separated, travel in opposite directions along the fibrils of 

 the achromatic spindle toward the centrosomes, around which they group them- 

 selves, and thus two star-like figures are formed, one at either pole of the 

 achromatic spindle. This is termed the diaster. 



3. Anaphase. The V-shaped daughter chromosomes now assume the form of 

 a skein or spirem, and eventually form the network of chromatin which is char- 

 acteristic of the resting nucleus. The nuclear membrane and nucleolus are also 

 differentiated during this phase. The cell-protoplasm begins to appear constricted 

 around the equator of the achromatic spindle, where double rows of granules are 

 also sometimes seen. The constriction deepens and the original cell gradually 

 becomes divided. 



4. Telophase. In this stage the cell is completely divided into two new cells, 

 each with its own nucleus, eentrosome, and centrosphere, which assume the 

 ordinary positions occupied by such structures in the resting stage. 



In the case of prickle-cells the subdivision of the cell is incomplete ; here the 

 achromatic spindle-threads appear to persist and bridge across the intercellular 

 spaces, constituting the prickles. 



The series of diagrams (Fig. 597), by Professor S. Deldpine, is intended to 

 explain the formation of some of the most important changes observed in nuclei 



