REPRODUCTION OF PROTOZOA. 107 



As it is by cell division that all embryos are formed from the egg, and 

 all growth is effected, the beginnings of this process are of much interest. 

 (a) Some very simple Protozoa seem to reproduce by what looks like 

 the rupture of outlying parts of the cell-substance. (6) The production 

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

 pods (e.g. Arcella, Pelomyxa) give off many buds at once, (c) Com- 

 moner, however, is the definite 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 

 liberating 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 — (a) 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, Protozoa 

 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 fertilisa- 

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

 there occurs at intervals a process of " conjugation " in 

 which two individuals unite either permanently or tem- 

 porarily. This is an incipiently sexual process ; it is the 

 analogue of the fertilisation of an ovum by a spermatozoon. 



(1) It is one of the recurrent phases in the life history of some of the 

 simplest Protozoa (Proteomyxa and Mycetozoa) (see p. 97), that a 



