FERTILIZATION IN PLANTS AND UNICELLULAR ORGANISMS 319 



process of fertilization in the Metazoa and the Metaphyta is almost 

 complete. 



But that the conjugation of unicellular organisms, like the 

 fertilization of multicellular organisms, is essentially a matter of 

 nuclear conjugation is shown more distinctly still by the ciliated 

 Infusorians, the most highly organized of the Protozoa. 



Here there is usually no complete union of the cell-bodies of the 

 two animals, but only an adhering of the apposed surfaces. In the 

 relatively large Paramcecium caudatum the process of conjugation 

 is very exactly known through the beautiful investigations of Maupas 



\J!IC 



Fig. 84. Conjugation and polar body formation in (he Sun-animalcule, Actinophfp 

 snl, after Scbaudinn. J, two free-swimming conjugated individuals, which in I/liave 

 become surrounded by a transparent gelatinous cyst. ITI, formation of the directive 

 spindles {RSp). IV, the polar bodies are formed {EK) ; A', the two sex-nuclei. F, these 

 are fused to form the conjugation-nucleus {K). VI, the conjugation-nucleus is trans- 

 formed into the division- spindle ; the polar bodies {RK) have penetrated the internal 

 cyst-wall, and are in process of degeneration. 



and E. Hertwig. In this case the mouth-surfaces of the two animals 

 come together and unite over a short area, and then the two animals 

 swim about together in this conjugated state. During this time very 

 remarkable changes take place in their nuclei. 



It is well known that these Infusorians have a double nucleus, 

 a large one, the macronucleus (Fig. 85, lua), and one which is usually 

 very small, the micronucleus (mi). We may ascribe to the former 

 of these the guidance and regulation of the everyday processes of life, 

 that is, briefly, of metabolism, and tlie preservation of the integrity 

 of the whole animal. The small nucleus has often been designated 

 the 'reproductive nucleus,' but as it plays no other part in repro- 



