HETEEOTYPE MITOSIS OF CELLS. 11 



descendants the power of division, or the capability of growth and function. If 

 it were not so, growth and function, or at least maintenance and function, would 

 continue uninterruptedly, and in the absence of accident or disease individual life 

 would continue for ever, and " old age" would be unknown. 



It appears, therefore, that the ancestors of certain tissue cells are capable of 

 producing only a certain number of descendants, which grow to the normal size and 

 perform their proper functions for a more or less fixed period, whilst in other cases 

 the power of division appears to be transmitted continuously, but the more remote 

 descendants become less and less capable of performing their proper functions. 

 The result in both cases is the same ; gradual decay, terminating in death. 



Heterotype Mitosis. In ordinary or homotype mitosis the chromosomes are 

 divided into equal parts, and, when the process of cell division is completed, each 

 daughter cell possesses the same number and same kind of chromosomes as the 

 mother cell from which it was derived (Figs. 3-8). In heterotype mitosis, the 

 number of chromosomes is reduced during the cell division, and each daughter cell 

 possesses only half the number of chromosomes that was present in the mother cell. 



The details of the division of the chromosomes during heterotype mitosis differ 

 in different groups of animals, but the end is the same in all cells in which the 

 process occurs, and is the reduction of the number of the chromosomes in the 

 daughter cells to half the number typical for the ordinary cells of the animal. 



The most typical form of heterotype mitosis is seen during the first maturation 

 division of many germ cells, in which, during the spirem or thread-like stage of the 

 chromatic substance, careful examination of the thread shows that it consists of 

 a number of alternate segments attached end to end, the number of segments 

 corresponding with the number of the chromosomes typical for the ordinary cells 

 of the animal. Towards the end of the prophase, the segments of the thread 

 become attached together in pairs which form a number of twin chromosomes. 

 These arrange themselves around the equator of the achromatic spindle, and it is 

 obvious that the number of twin chromosomes is only half the number of the 

 chromosomes originally present in the cell (Figs. 12 and 13). (See note 1, p. 79.) 



The total number of chromatic segments is still the same, for each twin 

 chromosome consists of two ordinary chromosomes attached side to side. 



The process of reduction takes place during the metaphase, when the two 

 segments of each twin chromosome become separated from one another. During 

 the anaphase the separated segments pass to the opposite poles of the achromatic 

 spindle, and when the telophase is completed the number of chromosomes in each 

 daughter cell is half that which was present in the mother cell (Figs. 12-19). 



In some cases, at the commencement of heterotype mitosis, the chromosomes 

 are not arranged in pairs as twins, but in groups of four, called tetrads, each tetrad 

 consisting of a pair of dyads. In those cases the two dyads of each tetrad are 

 separated from one another during the metaphase, and when the telophase is com- 

 pleted each daughter cell possesses only half the number of chromatic particles 

 which were present in the mother cell. 



It is known that a cell which contains only half the typical number of chromo- 

 somes can divide once, therefore from each original cell which underwent heterotype 

 mitosis four grand-daughter cells may be produced. It is still uncertain, however, 

 whether or not cells which contain only half the typical number of chromosomes 

 can further subdivide, or whether they can continue to live and function. So far 

 as the observations made can be relied upon, it appears that such cells either die 

 or they unite with another cell containing half the typical number of chromosomes 

 to prod uce a new cell which contains the typical number of chromosomes and which 

 possesses also the capability of reproducing itself by division. 



The Gametes. The gametes are the germ elements by whose union, in pairs, 

 new individuals are produced. 



They are of two kinds, female gametes or ova and male gametes or spermatozoa. 

 Both female and male gametes are modified cells, by means of which hereditary 

 characteristics are transmitted from generation to' generation, and they are derived 

 from cells called primitive germ cells, whose origin will be considered in association 

 with the development of the germinal layers. 



