THE CELL AND CELL PROLIFERATION. 3 



die, but may live for some time and show active motile powers. Such a de- 

 nucleated cell has, however, lost two of its most important functions: (i) its 

 power of constructive metabolism; that is, of taking up nutritive material from 

 without and building this up into its own peculiar structure the power of 

 repair; and (2) the power of reproduction. For these reasons the nucleus has 

 been considered as especially presiding over these two cell functions. 



The CENTROSOME is a small spheroidal body usually found in the cytoplasm 

 near the nucleus, less commonly within the nucleus. In actively dividing cells 

 the centrosome is frequently double, having divided in anticipation of the succeed- 

 ing cell division. In Invertebrates the centrosome sometimes shows a reticular 

 structure and contains a still more minute body, the centriole. For further 

 details concerning the centrosome, see Mitosis, p. 4, 



In the simplest forms of animal life a single cell, such as has been described 

 above, constitutes the entire individual, and as such is capable of performing 

 the functions which are recognized as characteristic of living organisms metab- 

 olism, irritability, motion, reproduction and special functions. The develop- 

 mental history of such an individual is extremely simple. The nucleus under- 

 goes division and this is accompanied or followed by division of the cytoplasm. 

 The single cell thus becomes two cells, similar in all respects to the parent cell. 



In all higher, that is multicellular animals, however, the different functions 

 are distributed specifically to different cells and these cells are specifically 

 differentiated morphologically for the performance of these different functions. 

 There is, therefore, not the simple division of a parent cell to form two similar 

 daughter cells, each constituting an individual, but a differentiation from the 

 single original germ cell, the fertilized ovum, of many different kinds of cells, 

 and their specialization to 'form the various tissues and organs which constitute 

 the adult body. 



CELL DIVISION. 



In the development of the embryo, cell division of course succeeds fertiliza- 

 tion. A proper understanding, however, of the changes which take place in 

 the ovum and in the spermatozoon previous to fertilization requires the con- 

 sideration of cell division at this point. 



Two types of cell division are recognized : (i) direct cell division or amitosis 

 and (2) indirect cell division or mitosis. 



(i) Amitosis (Fig. 2). In this form of cell division there is no formation of 

 spindle or of chromosomes (see IVJitosis, p. 4), the nucleus retaining its reticular 

 structure during divisipn. There is first a constriction of the nucleus, followed 

 by complete division into two daughter nuclei. During the division of the 

 nucleus a constriction appears in the cytoplasm. This increases until the 

 cytoplasm is divided into two separate masses (daughter cells), each containing 



