3 -'4 



ANIMAL DEVELOPMENT 



active half and passive half. The active half, in each individual, 

 migrates into the other Animalcule, and fuses with the passive 

 half there present. After this interchange of nuclear material, 

 which constitues the essential part of the process, the two con- 

 jugating individuals separate, each of them containing what may 



Fig. 848. — Temporary Conjugation of a Slipper-Animalcule [Param<£ciinn caudatum) 



Diagram to sliow successive divisions, &c., of micronuclei. Fragments which disappear represented by 

 clear circles, and fragments (of the compound nuclei) which become macronuclei shown as horizontally 

 shaded circles. Read from below upwards. The changes during conjugation (c) of the two animalcules 

 (l and 11) are shown below ab: « and /, active and passive nuclei, the migration of the former indicated by 

 dotted lines. The changes after conjugation shown above ab: at Di, each conjugated animalcule has divided 

 into two, and each of these (at D2) has again divided, so that there are now eight new animalcules (1-8). 



be called a compound or fertilization nucletis. The fusion of 

 two nuclear masses, derived from different individuals, i.e. fer- 

 tilisation, is followed by increased activity in the vital processes. 

 By it the living substance with which life is associated is, so to 

 speak, stimulated and freshened, or, to use a more technical 

 expression, rejiivenesced. Why this should be the case is still 

 unknown. In the Slipper- Animalcule conjugation is followed 

 by fission, a mark of increased activity. And it has been shown 

 that without occasional conjugation between individuals not too 



