Ii6 PHYLUM PROTOZOA THE SIMPLEST ANIMALS. 



REPRODUCTION OF PROTOZOA 



Growth and reproduction are on a different plane from 

 the other functions. Growth occurs when income exceeds 

 expenditure, and when constructive or anabolic processes 

 are in the ascendant. Reproduction occurs at the limit of 

 growth, or sometimes in disadvantageous conditions. 



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, (b] The production 

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

 pods (e.g. Arcella, Pelomyxd] 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 (&) the 

 fertilisation of ova by spermatozoa. As Protozoa have 

 no "body" though the beginnings of one are seen in 

 the colonial forms they cannot be said to exhibit sexual 

 reproduction in the first sense (#), yet many of them 

 (especially the Sporozoa) give origin by division to special 

 reproductive cells. And although many Protozoa can live 

 on, dividing and multiplying, for prolonged periods without 

 the occurrence of anything like fertilisation, processes 

 corresponding to fertilisation are of general occurrence. 

 For in many of the Protozoa there occurs at intervals a 

 process of " conjugation " in which two individuals unite 



