SEXUAL REPRODUCTION. 



T5I 



ing the origin of fertilisation. Just as the very beginning of 

 reproduction may be detected in the almost mechanical break- 

 age of a form like Schizogeiies, so the very beginning of fertili- 

 sation is found in the almost mechanical flowing together of 

 exhausted cells. 



Diagrammatic representation of the stages 

 in the origin of fertilisation, — (I.) Plas- 

 modium ; (IL) multiple conjugation; 

 (III.) ordinary' conjugation; (IV.) con- 

 jugation of dimorphic cells ; (V.) fertilis- 

 ation of ovum by spermatozoon. 



{6) Between this and the process usually described as con- 

 jugation, there are some interesting links. Sometimes as many 

 as three or four spores of lowly Algae club together, as if to 

 gather sufficient momentum to make a combined start in life. 

 The young forms of the sun-animalcule {Actinosphcerium) 

 usually unite in twos, but Gabriel has observed in some cases 

 a multiple union. So in gregarines (common parasites in 

 invertebrates), while the usual union is certainly dual, Gruber 

 has again observed what may be termed multiple conjugation. 

 Union of three has also been observed as an exception in 

 several infusorians. The union of more than two may thus be 

 interpreted as intermediate between the formation of plasmodia 

 and the normal dual conjugation. 



{c) Conjugation of two .s-/;;;//(7r unicellular organisms occurs, 



