546 THE HISTORY OF BIOLOGY 



the nucleus, which then displays a number of different phenomena in differ- 

 ent forms: on the one hand, direct division through interlacing; on the other 

 hand, a regular mitosis, with a division of the centrosome and spindle- 

 formation; and, between the extremes, a number of transition forms. After 

 the division the nucleus and plasma grow at different speeds until a certain 

 ratio of bulk arises between them; then a fresh division takes place. This 

 "nucleus-plasma relation," as R. Hertwig calls it, is thus a decisive factor 

 in reproduction and not, as formerly supposed, a growth beyond the nor- 

 mal standard, for this can vary considerably even in the same species. Still 

 more remarkable are the phenomena that R. Hertwig discovered upon the 

 conjugation of the Protozoa — in the fusion of two individuals which pre- 

 cedes division in certain circumstances. In many Protozoa there exists, besides 

 the ordinary large nucleus, a small nucleus, called the micronucleus, which, 

 previous to conjugation, divides itself twice; three of the divided nuclei per- 

 ish, while the fourth unites with the corresponding nucleus in the conju- 

 gating neighbouring cell, whereupon out of the unifying product fresh nuclei 

 are formed in the cells, whose large nuclei meanwhile disintegrate. In the 

 three disappearing divided nuclei R. Hertwig has seen the equivalent of the 

 polar bodies in the eggs of higher animals, while the likewise moribund 

 large nucleus has been held to correspond to the body in a higher animal, 

 which dies, whereas the sexual cells, here equivalent to the conjugated 

 small nuclei, reproduce the life-form. Whether or not these comparisons 

 may perhaps have been carried too far, the future must decide; it is certain 

 that through them a number of vital phenomena of general interest have 

 been viewed in an entirely new light, and the uniformity of the fundamental 

 phenomena of life has received further confirmation. 



Bacteriology 

 Of even greater importance, however, has been the progress made in the 

 sphere of bacteriology; it was during this period that light was thrown 

 upon the part played by bacteria as producers of disease and that their bi- 

 ology was discovered. Theories had long been in circulation that minute 

 living "seeds of disease" were the causes particularly of the great plagues; 

 such a hypothesis had been set up during the Renaissance by the Italian 

 physician Girolamo Fracastoro (1483-1533); Linnsus, it will be remem- 

 bered, had embraced similar ideas; these theories had been encouraged by 

 the discovery of yeast-fungi in the eigh teen-thirties; Henle had been spe- 

 cially interested in "parasites" as producers of disease, and as a proof of his 

 assumption of such a cause of disease he had formulated the principle : con- 

 stant existence, isolation from foreign interference, reproduction of the form 

 of disease by means of the isolated parasite. These conditions, however, 

 were found to be difficult to fulfil; even Pasteur, who was nevertheless the 

 founder of modern bacteriology, had not succeeded in finding a means of 



