GENERAL EMBRTOLOGT. 139 



II. GENERAL EMBRYOLOGY. 



Origin of Organisms. — Since the development of every indi- 

 vidual begins with an act of generation, the ways by which new 

 organisms may arise should be mentioned first in this chapter. 

 If we wish to limit ourselves to that which has been actually 

 observed, we must still cling to the old expression of the renowned 

 Harvey, " Omne vivum ex ovo," and modifying it somewhat say, 

 Omne vivum e vivo : that every living organism is derived from 

 another living organism. "We must limit ourselves to the mode of 

 origin which has been termed tocogony, or generation by parents. 

 The great importance which the question of generation without 

 parents, or spontaneoiis generation, has obtained through the 

 evolution theory renders a consideration of this question necessary 

 at this point. 



I. Gesteratio Spontaitea, Akchegoxy. 



Theory of Spontaneous Generation. — The old zoologists, even Aristotle 

 himself, believed that many animals, including even highly organized 

 forms, like frogs and most insects, arose by spontaneous generation from 

 the mud. Not until the seventeenth and eighteenth centuries did this 

 doctrine find energetic opponents, in Spallanzani, Francesco Redi, Rosel 

 von Rosenhof, Swammerdam, and others, who endeavored to prove 

 experimentally that all animals lay eggs whioli must be fertilized by the 

 spermatozoon in order to develop further. By their convincing investi- 

 gations the doctrine of spontaneous generation was driven into the realm 

 of the lower animals. Here it found a new foundation in the occurrence 

 of parasites inside of animals which, at the beginning of their life, without 

 doubt must have been free from these internal inhabitants. Parasitolo- 

 gists maintained that the parasites arose quite anew from the superfluous 

 plastic material of their host. At last, by a series of epoch-making 

 researches, the way was discovered by which the young of the parasite, 

 developing from eggs, find their way into the body of their host. It was 

 until recently considered a proof of the doctrine of spontaneous generation 

 that, after a time, animal and pjlant life (unicellular organisms, infusorian 

 animalcules, etc.) appears in water supposed to contain no living thing 

 whatever ; further, that organic fluids became foul by the development of 

 the lowest of the plants, the bacteria. At present we know that in all 

 these cases germs of organisms, carried about by the air, are the cause of 

 the new development of life. If the germs be killed by heating the 

 glass and boiling the fluid, and if by suitable means the entrance of new 

 germs be prevented, then such a ' sterilized fluid ' remains permanently 

 unchanged. It has been found, indeed, that spores, particularly of bac- 

 teria, have an extreme power of resistance, and in many cases must be 



