PROTOZOA 127 



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

 union of two nuclei of opposite sex. In the metazoa the nuclei of the 

 germ cells undergo before they are ready to unite repeated divisions, 

 in which the number of the chromosomes is reduced to one-half the 

 usual number. 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 chromosomes. The number 

 of chromosomes which are formed during mitosis is constant and char- 

 acteristic 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 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. 



