GENERAL ANATOMY 



59 



these animals be cut into nucleate and anucleate halves, the first lives and 

 regenerates the lost parts; the anucleate portion moves about for a time, 

 apparently as long as the stored energy lasts, but it cannot assimilate or 

 reproduce the missing portions and so sooner or later it dies. 



The Centrosome. Besides the nucleus there frequently occurs a 

 special body in the protoplasm, the centrosome, which, on account of its 

 small size and a behavior similar to achromatin with reference to staining- 

 fluids, was long overlooked. It is well distributed among the Metazoa, 

 but is absent from most Protozoa, in many of which it appears only at 

 certain times and then disappears. It is probable that it is a derivative of 

 the nucleus, a part of the achromatin which has left the nucleus; in other 

 cases possibly a second nucleus which by degeneration has lost the chro- 

 matin and retained only the motor nuclear substance, the achromatin. 

 In its function the centrosome is a specific organ of cell division which 

 controls both the division of the nucleus and that of the cell itself. 



Multiplication of Cells. Increase in cells occurs exclusively by 

 division or by budding (gemmation). Most common is binary division 

 in which a circular furrow appears 

 on the surface of the cell, deepens 

 and cuts the cell into two equal 

 parts. Mutiple division is more 

 rare and can only occur in multi- 

 nucleate cells. Here the cell 

 divides simultaneously into as 

 many (sometimes hundreds) 

 daughter-cells as there were nuclei 

 present. In all forms of division 

 the similarity of the products is 

 characteristic, while in budding 

 the resulting parts are unequal, 

 one or more smaller daughter- 

 cells, the buds, being constricted from a large mother-cell (fig. 21). 



Direct Cell Division. Every cell division is accompanied by nuclear 

 division or nuclear division has previously occurred. Direct and indirect 

 division are recognized. Direct division is most common in Protozoa, 

 especially in nuclei with abundant chromatin (figs. 21, 120, 150, 155). 

 The nucleus elongates and is divided by constriction, in the same way 

 that the cell itself constricts. Since the protoplasm has no special arrange- 

 ment for dividing the nucleus (the latter besides protected by its mem- 

 brane), we must conclude that the nucleus divides itself. The dividing 

 force resides in the achromatic framework, which correspondingly often 



FIG. 21. Cell budding. Pcdjphrya geni- 

 mipara with buds (a) which separate and 

 form free young (b). N, nucleus. 



