$6 REPRODUCTION AND LIFE HISTORY. 



males ; or by contrasting the secondary sexual characters 

 of the two sexes. 



Stages in the history of fertilisation. While it is not difficult 

 to see the advantage of fertilisation as a process which helps to sustain 

 the standard or average of a species and as a source of new variations, 

 we can at present do little more than indicate various forms in which 

 the process occurs. 



(a) For/nation of riasinodia, the flowing together of numerous feeble 

 cells, as seen in the life-history of those very simple Protozoa 

 called Proteomyxa, e.g. Protoinyxa, and Mycetozoa, e.g. flowers 

 of tan (^'l 1 ] thai in in septicnin}. 



(l>) Multiple conjugation, in which more than two cells unite and fuse 

 together, as in some Sporozoa and in the sun-animalcule 

 ( A ctinosplucrinin ) . 



((') Ordinary conjugation, in which two similar cells fuse together, 

 observed in Sporozoa and Rhizopods. In ciliated Infusorians, 

 the conjugation may be merely a temporary union, during which 

 nuclear elements are interchanged. 



(d] Dimorphic conjugation, in which two cells different from one 



another fuse into one, a process well illustrated in Vorticella 

 and related Infusorians, where a small, active, free-swimming 

 (we may say, male) cell unites with a fixed individual of normal 

 size, which may fairly be called female (see Fig. 40, p. 94). 



(e) Fertilisation, in which a spermatozoon liberated from a Metazoon 



unites intimately with an ovum liberated from another individual 

 normally of the same species. 



Divergent modes of sexual reproduction. (a) Herm- 



aphroditism is the combination of male and female sexual 

 functions in varying degrees within one organism. It may 

 be demonstrable in early life only, and disappear as male- 

 ness or femaleness predominates in the adult. It may 

 occur as a casualty or as a reversion ; or it may be normal 

 in the adult, e.g. in some Sponges and Coelentera, in many 

 "worms, "such as earthworm and leech, in barnacles and 

 acorn- shells, in one species of oyster, in the snail, and in 

 many other Bivalves and Gastropods, in Tunicates and in 

 the hag-fish. In most cases, though these animals are 

 bisexual, they produce ova at one period and spermatozoa 

 at another (dichogamy). It rarely occurs (e.g. in some 

 parasitic worms) that the ova of a hermaphrodite are fertilised 

 by the sperms of the same animal (autogamy]. Certain 

 facts, such as the occurrence of hermaphrodite organs as 

 a transitory stage in the development of the embryos of 

 many unisexual animals (e.g. frog and bird), suggest that 



