230 LECTURES ON BIOLOGY 



rudiment (Anlage) slowly originate, by division, differentiation, and 

 growth, the embryo, and finally the mature organism. The reason 

 why in the multi-cellular organisms a conjugation of the entire 

 animals can no longer take place is that only naked cells can 

 become fused together. A further important difference is that the 

 two copulating sex-cells, which for that reason we describe as male 

 and female germ-cells or as spermatozoon and ovum, differ greatly 

 in size, form, and appearance, while the Paramcecia which proceed 

 to a union are perfectly alike. On the other hand we know 

 already among infusorians a separation into 'male ' and ' female ' 

 sexes, and the similarity in the phenomena in the highest animals 

 is increased by the further fact that the union of conjugating 

 individuals is permanent. 



If we examine the numerous minute crustaceans which 

 are among the most familiar inhabitants of the fresh-water 

 aquarium we shall find their shells sometimes covered with a 

 dense, woolly fungus which under a magnifying-glass are dis- 

 covered to be colony-forming infusorians, members of the genus 

 Vorticella. Dainty, many-branched little trees carry at the ends 

 of their branches minute bells, the animalcules proper. In con- 

 trast to Param&cium, the cilia in Vorticella are greatly reduced 

 in number; the entire stalk is naked, only one well-developed 

 spiral fringe leading to the mouth-opening at the anterior end. 

 Its function is to waft, by its lashing movement, the food to the 

 mouth. In some species we observe, in addition, a simple circle 

 of cilia round the body. As Vorticella generally does not change 

 its habitat a complete cilia-dress is no longer of any use to them. 



Like all other infusorians, no bell-animalcule is able to repro- 

 duce by simple fission, but as the drawing-out or constricting 

 process i& in numerous forms incomplete, and as the daughter- 

 individuals remain connected with the mother-body by means of 

 a thin stalk proceeding from the posterior end, this frequently 

 gives rise to the formation of large colonies. When the time of 

 conjugation arrives the individual citizens of the Vorticella colony 

 exhibit remarkable differences. While one part remains un- 

 changed, others in quick succession divide each into four indi- 

 viduals, without waiting for growth to make up the resulting 

 decrease in size. After a short time these little ' male ' germs 

 separate from the mother-stem and swim freely about by means 



