180 PRTNCIl'LES OF ANIMAL BIOLOGY 



exchange of micromiclei and the foi-ination of a new nucleus from the two 

 parts. The repeated divisions of the cells following conjugation arc to 

 be likened to segmentation of the fertilized ovum of the metazoa. 



In Vorticella and its allies there is a series of nuclear changes some- 

 what like that in Paramecium, but there is complete fusion of a smaller 

 with a larger individual. 



Possible Maturation in Protozoa. — In most Protozoa there is no 

 process which can be considered analogous to the process of maturation 

 of the ovum and sperm of the metazoa. The chromosomes in the 

 Protozoa are so small and their number is so great that it is not possible 

 to count them to determine whether reduction in number has taken place. 

 In a few species of Rhizopoda and Sporozoa a portion of the nucleus of 

 each individual is known to be cast out into the cytoplasm where it 

 degenerates. A number of phenomena once interpreted as reduction 

 are now regarded as indicating degeneration. In Paramecium and 

 other Infusoria there is a loss of nuclear material, but it has not been 

 shown conclusively that there has been a reduction in the number of 

 chromosomes. 



Significance of Fertilization in Protozoa. — Because of the diversity 

 of conditions under which sexual reproduction in the Protozoa occurs, 

 and the variety of arrangements for its accomplishment, the function 

 of fertilization^ in this group has been variously interpreted. When it 

 occurs regularly and involves specialized gametes, it may appear to be of 

 fundamental importance; but when it occurs only at rare intervals fer- 

 tilization seems unessential. The view has long been held that fertiliza- 

 tion in Protozoa, as in metazoa, results in rejuvenation, or a renewal of 

 vigor. This view has been seriousl}^ questioned, however, in the light 

 of evidence which on the one hand appears to indicate that sexual 

 reproduction in certain Protozoa is not indispensable and on the other 

 hand shows that it does not necessarily result in reinvigoration. Further 

 discussion of a question so much in doubt would scarcely be profit- 

 able here. 



Parthenogenesis. — In an earlier part of this chapter the statement 

 was made that sexual reproduction involves two parents and the fusion 

 of two germ cells. Nevertheless it is not uncommon to find species of 

 invertebrates among which, for considerable periods of time, no males 

 can be found. Eggs, however, are produced by the females, and these 

 develop into new individuals like the parent, although fertilization does 

 not occur, since no males are present. By their origin and maturation 

 the cells giving rise to new individuals are ova, hence the method is ob- 



' 1 The term fertilizaiion is properly used only to designate the union of egg and 

 sperm among higher organisms; but to emphasize the analogy between similar proc- 

 esses in complex and simple animals, it is applied here to the union of gametes in the 

 Protozoa. 



