130 MORPHOLOGY AND CULTURE OF MICROORGANISMS 



In protozoa, as in metazoa, the essential process in fertilization is the 

 union of two nuclei of opposite sex. In dividing, cells may go through 

 a process called mitosis during which the chromatin of the nucleus is 

 grouped into more or less rod-shaped masses which are called chromo- 

 somes. The number of chromosomes which are formed during mitosis 

 is constant and characteristic for each species. In the reproductive 

 areas, during the two divisions just preceding the maturity of cells 

 which are to become ova or spermatozoa, the number of chromosomes is 

 reduced to exactly one-half of the number which are formed during the 

 division of cells outside of the reproductive areas of the same animals. 

 The process by which the number of chromosomes is reduced to one-half 

 is termed chromatic reduction, and the fragments of chromatin which in 

 the female are unused and which are extruded from the cell during the 

 process are called polar bodies. While reduction in the number of 

 chromosomes has been shown to occur prior to fertilization in a number 

 of the protozoa, in many species a more primitive process consisting of 

 the mere extrusion of masses of chromatin irrespective of the number of 

 chromosomes is found to occur. It is evident that the chromatin is, 

 at least usually, reduced in amount preparatory to the sexual process. 



Although in certain of the protozoa nuclear division is accomplished 

 by a process of mitosis similar to that which occurs in multicellular 

 animals, in many it is affected by a much more primitive process. 

 The nucleus may be resolved into scattered granules of chromatin- 

 chromidia which may subsequently become reconstructed into a num- 

 ber of nuclei. The nucleus may divide by direct division, that is, by sim- 

 ple constriction into two approximately equal parts. Between this form 

 of division and the classical mitosis there is every possible transition. 

 The centrioles or centrosomes are frequently intranuclear in the 

 protozoa. In the case of primitive nuclei without definite nuclear mem- 

 brane a division simulating mitosis is termed promitosis. In other 

 forms in which there is a nuclear membrane but in which the centrioles 

 remain intranuclear throughout division, the process is called meso- 

 mitosis. The nuclear membrane often persists throughout division 

 and the chromosomes are in many forms very minute or are not 

 definitely formed. 



The fertilizing processes which occur in the protozoa may be grouped 

 under three heads: Copulation, Conjugation and Self-fertilization. In 

 copulation two whole cells unite. The cells taking part in this union 



