74 MORPHOLOGY AND CULTURE OF MICROORGANISMS. 



grouped into masses which are called chromosomes. The number of 

 chromosomes which are formed during mitosis is constant and character- 

 istic for each species. In the reproductive areas, during the two divisions 

 just preceding the maturity of cells which are to become ova or spermat- 

 ozoa, the number of chromosomes is reduced to exactly one-half of the 

 number which are formed during the division of cells outside of the re- 

 productive areas of the same animals. The process by which the number 

 of chromosomes is reduced is reduction, and the fragments of chromatin 

 which 'are unused and which are extruded from the cell during the process 

 are called polar bodies. Reduction and extrusion of polar bodies always 

 precede fertilization (Fig. 46). 



The fertilizing processes which occur in the protozoa may be classed 

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

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

 called gametes and they are male micro gametes, or female macro gametes. 

 The cells which produce the gametes are called gametocytes. The product 

 of the union is called a copula or zygote. If the uniting cells be equal 

 in size the copulation is isogamous; if they be unequal, the copulation is 

 said to be anisogamous. Anisogamous copulation, the union of two 

 unequal cells, is most typically -seen in the fertilization of a large macro- 

 gamete by a small microgamete. Copulation is a most important fertiliz- 

 ing process among the pathogenic protozoa. Conjugation, the second 

 method of fertilization, only occurs among the ciliata. In it, two adult 

 individuals are placed in apposition. The nucleus of each cell* first re- 

 duces and then divides into two halves, one male, the other female. 

 Each organism retains its female half nucleus, while an exchange of the 

 male half nuclei is effected. Processes of self-fertilization, such as 

 autogamy and parthenogenesis, are included under the third heading. In 

 autogamy the nucleus of a single cell divides into two parts. Each of these 

 undergoes two further divisions, during which the chromosomes are re- 

 duced and polar bodies are extruded. The two resulting, reduced, half- 

 nuclei unite, always in the same cell, to form a new nucleus. Partheno- 

 genesis is the development of new individuals from a female cell without a 

 preceding fertilization ; this process occurs in many protozoa, and through 

 it may be explained some of the acute outbreaks of malaria which may 

 occur in patients who once suffered from that disease and were thought 

 .to have been cured of it. 



It was said that fertilization and multiplication might be distinct 



