HIS TOR Y OF FER TILISA TION. 5 1 



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plasmic life of the creature. We believe that the contrast 

 between the sexes gives almost universal expression to this 

 fundamental alternative of variation. 



This theory may be confirmed in many ways, e.g., by 

 contrasting the characteristic products of female life, — passive 

 ova, with the characteristic products of male life, — active 

 spermatozoa ; or by comparing the complex conditions 

 (such as abundant food, favourable temperature) which tend 

 to produce female offspring with the opposite conditions 

 which tend to produce males ; or by contrasting the secondary 

 sexual characters of males {e.g., bright colours, and smaller 

 size), with the opposite characteristics of females. 



Stages in the History of Fertilisation. 



(a) Formation of Plasmodia, the flowing together of 

 numerous feeble cells, as seen in the life-history of 

 those very simple Protozoa called Proteomyxa, e.g., 

 Protomyxa, and Mycetozoa,^.^., flowers of tan {^thul- 

 ium septicuni). 



{b) Multiple Conjugation, in which more than two cells 

 unite and fuse together, a process said to occur in 

 some Gregarines, in the sun-animalcule (Actinosphcer- 

 ium), and in some simple Algge. 



(c) Ordinary Conjugation, in which two similar cells fuse 



together, observed in Gregarines, Rhizopods, and 

 many Algse. In ciliated Infusorians, the conjugation 

 is 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 illus- 

 trated in Vorticella and related Infusorians, where a 

 little, active, free-swimming (we may say, male) cell 

 unites witli a fixed individual of normal size, which 

 may fairly be called female. 



(e) Fertilisation, in which spermatozoa liberated from a 



Metazoon unite intimately with ova liberated from 

 another individual of the same species. 



Divergent Modes of Sexual Reproduction, 

 (a) Hermaphroditism is the union of male and female 



