FERTILIZATION IN PLANTS AND UNICELLULAR ORGANISMS 317 



organisms, which we have already studied, had not made it clear to us 

 that there are two associated processes, quite different in nature, the 

 conjugation of unicellular organisms would have led us to that 

 conclusion. It has long been known that two unicellular plants or 

 animals occasionally become closely apposed and fuse ; and this process 

 of ' conjugation ' was many years ago regarded as an analogue to 

 ' fertilization,' although it is only through the laborious investigations of 

 the last two or three decades that this supposition has been proved to be 

 correct. We now know that a process quite analogous to that which 

 we have learnt to know 

 as ' fertilization ' takes _ -^ g 



place among unicellulars, '^ ^"^ '' - — ^ -^'^ 



only in this case it is not 

 directly connected with 

 i-eproduction and multi- 

 plication, but occurs in- 

 dependently of them, 

 and, in its most primitive 

 form, it results, not in 

 an increase but — for a 

 short time at least — in 

 ■ a diminution of the num- 

 ber of individuals. This 

 occurrence of the process 

 independently of repro- 

 duction appears to me 

 of inestimable value theo- 

 retically, for it frees us 

 completely, from the old 

 deep - rooted preconcep- 

 tions in the interpretation of fertilization. 



First let us briefly sketch the ^process itself in the main forms of 



its occurrence. 



The most primitive form of conjugation is undoubtedly the com- 

 ' plete fusion of two unicellular organisms of the same species, as 

 we see it to-day in . unicellular plants, and also among the lowest 

 unicellular animals, such as the flagellate Infusorians, Gregarines, and 

 Rhizopods. It is well seen, for instance, in the .NoctiluciB, those 

 unicellular flagellate organisms which cause the familiar marine phos- 

 phorescence extending uniformly over wide surfaces of water (Fig. 83). 

 In these forms Prof. Ischikawa of Tokio was able to trace the whole 

 process of conjugation. To begin with, two Noctilucas range them- 



Fig. 83. Conjugation of Noctiluoa, after Ischikawa. 



A, two Noctilucas beginning to coalesce ; pr, the proto- 

 plasm drawn out into processes which traverse the 

 gelatinous substance of the cell ; fe, the nucleus. 



B, the cells and their gelatinous substance have fused ; 

 the nuclei, in which the chromosomesare visible, are 

 closely appcsed ; CK, centrospheres. C, the two nuclei 

 are united in one nuclear spindle ; beginning of 

 division. D, completion of the division. Highly 

 magnified. 



