MODES OF SEXUAL REPRODUCTION IN PROTOZOA 533 



of a special characteristic arrangement of the chromatic substance 

 the process is called amitotic division; when the chromatin arranges 

 itself in a very definite manner in bands or rods, which become equally 

 divided, the process is known as mitotic division, karyokinesis , or 

 karyomitosis. When this form of division (quite common in protozoa) 

 occurs the kinetic achromatic substance of the nucleus arranges 

 itself in the form of an achromatic spindle, with a very small granule 

 or point, the centrosome, at either end. The chromatic substance 

 at the same time forms a definite number of threads, bands or rods, 

 which divide by splitting into double the number originally present. 

 They then move in equal numbers toward the centrosomes, so that 

 at the end of division each nucleus possesses again the same number 

 of chromosomes as the original nucleus when it was ready to divide 

 and its membrane began to be dissolved. The number, type, and 

 character of chromosomes is the most constant morphologic part of 

 a cell, and all cells of the same species have the same number of 

 chromosomes which are considered as the carriers of all the elements 

 transmissible by heredity. 1 When two germ cells, however, unite to 

 form a new being the number of chromosomes in each, by a process 

 of maturation and by the expulsion of chromosomes into polar bodies, 

 becomes reduced to one-half, so that the new cell formed by the 

 union of two mature germ cells has not double the original constant 

 number of chromosomes. Several types of cell division occur among 

 protozoa. There is simple binary division with splitting into two 

 equal parts or new cells. This is the type generally found among 

 the cells of higher metazoic animals. Budding as seen in the budding 

 fungi or yeast cells is also encountered, or there may be a splitting up 

 of the cell into a number of smaller daughter cells, each receiving its 

 proportionate share of the nucleus. The nuclear changes in protozoa 

 may closely resemble and be as complicated as the karyokinesis in 

 higher cells, or the granular chromatin may divide as such without 

 solution of the nuclear membrane, which may simply become con- 

 stricted in the middle and finally be cut in two at this point. The 

 cells of higher animals when dividing generally assume a simple 

 globular form, but in protozoa division is found in fully differen- 

 tiated cells, for instance, in the flagellate trypanosomes. Starting 

 with the blepharoblast (micronucleus, root of the flagellum) all the 

 structures of the organism, including the flagellum, undulating 

 membrane, macronucleus, and cytoplasm, become split in two some- 

 times equal, sometimes unequal, masses. 



Different Modes of Sexual Reproduction in Protozoa. In the phylum 

 protozoa a variety of types of reproduction are encountered. 



1. Autogamy, or Automyxis. In the fertilization by autogamy, or 

 automyxis, which is widespread among protozoa, there occurs an 



1 It is, however, almost certain that the cytoplasm likewise contains certain constant elements 

 always propagated in multiplication, which are likewise the carriers of hereditary properties 

 of the living substance. 



