KEPR OD UC riON. 1 1 1 



Protozoa seem to reproduce by what looks like the rupture 

 of outlying parts of the cell substance, {b) The production 

 of a small bud from a parent cell is not uncommon, and 

 some Rhizopods {e.g., Arcella, Pelomyxa) give off many 

 buds at once, (c) Commoner, however, is the deiinite and 

 orderly process by which a unit divides into two — ordinary 

 cell division, {d) Finally, if many divisions occur in rapid 

 succession or contemporaneously, and usually within a cyst 

 enclosing the parent cell, i.e., in narrowly limited time and 

 space, the result is the formation of a considerable number 

 of small units or spores. In the great majority of cases, 

 each result of division is seen to include part of the parent 

 nucleus. 



A many celled animal multiplies in most cases by liberat- 

 ing reproductive cells — ova and spermatozoa — different 

 from the somatic cells which make up the " body." A 

 Protozoon multiplies by dividing wholly into daughter cells. 

 This difference between Metazoa and Protozoa in their 

 modes of multiplication is a consequence of the difference 

 between multicellular and unicellular life. Each part of a 

 divided Protozoon is able to live on, and will itself divide 

 after a time, whereas the liberated spermatozoa and ova of 

 a higher animal die unless they unite. 



By sexual reproduction, we mean («) the liberation of 

 special reproductive cells from a " body," and {b) the 

 fertilisation of ova by spermatozoa. It is obvious that 

 unicellular Protozoa can show nothing corresponding to 

 sexual reproduction in the first sense. Moreover, Pro- 

 tozoa can live on, dividing and multiplying, for prolonged 

 periods without the occurrence of anything like fertilisation. 



So it is often stated as a characteristic of Protozoa that 

 " they have no sexual reproduction." But if this mean 

 that the unicellular Protozoa have no special reproductive 

 cells, then it is a truism. If, however, the statement mean 

 that the Protozoa are without anything corresponding to 

 fertihsation, then it is not true. For in many of the 

 Protozoa, there occurs at intervals a process of " conjuga- 

 tion " in which two individuals unite either permanently or 

 temporarily. This is an incipiently sexual process ; it is 

 the analogue of the fertilisation of an ovum by a spermato- 

 zoon. 



