THE MICROSCOPE IN BIOLOGY. 125 



The doctrine of spontaneous generation or aUogenesis 

 has been the object of considerable research, but the bril- 

 liant experiments of Pasteur have shown that when all 

 access of living organisms into fluids is prevented, no de- 

 velopment of such organisms can be proved in any case 

 to occur. If the access of air, for instance, to a liquid 

 which has been boiled, is filtered through a plug of cotton- 

 wool, no living forms will appear in the liquid, but on 

 examination, such forms will be found in considerable 

 numbers in the cotton-wool, proving the presence of these 

 forms or their germs in the external air. Recent experi- 

 ments also render it probable that some cell-germs are 

 indestructible by a heat far exceeding that of boiling 

 water. 



There are three forms of cell-multiplication, by fission, 

 by germination or budding, and by internal division. The 

 latter mode is termed endogenous. In it new cells are 

 produced within a parent-cell by the separation of the 

 bioplasm into a number of distinct masses, each of which 

 may become a new cell, as in the fecundated ovum. 



Fission, or the division by cleavage of a parent-cell- into 

 two or four parts, may be regarded as a modification of 

 endogenous cell-multiplication. A good example of it 

 may be seen in cartilage. 



Budding or germination consists in the projection of a 

 little process or bud from the mass of bioplasm, which is 

 separated by the constriction of its base, and becomes an 

 independent cell. 



8. Reproduction in the higher organisms consists essen- 

 tially of the production of two distinct elements, a. germ- 

 cell or ovum, and a sperm-cell or spermatozoid, by the 

 contact of which the ovum is enabled to develop a new 

 individual. Sometimes these elements are produced by 

 different parts of the same organism, in which case the 

 sexes are said to be united, and the individual is called 

 hermaphrodite, androgynous, or monoecious. In other in- 



