REPRODUCTION I 5 5 



senting research achievements, and someone can then build up the true 

 picture for us to see. 



REPRODUCTION * 



EMANUEL R ADL 



I. MODES OF REPRODUCTION 



One of the most important and characteristic features which differ- 

 entiates the Hving from the non-Hving, is the power of reproduction. No 

 organism is formed by the action of material forces, but each one is pro- 

 duced by a living predecessor. From the time of Aristotle, however, there 

 have always been some who have maintained this assumption to be in- 

 correct, and that there are certain circumstances under which spontaneous 

 generation may take place. Moreover, the method by which life arises 

 from life, a phenomenon without analogy in inorganic nature, presents a 

 problem which is just as obscure in the lowest types of life as in mankind. 



Life renews itself in two ways: the first sexually, when two individuals 

 are essential for the production of the offspring; the second asexually, 

 when one individual alone can produce another. 



In asexual reproduction a smaller or larger portion of the body 

 separates itself from the mother organism, and by growth and differentia- 

 tion develops into a new individual. This individual exists side by side with 

 the mother organism, which, in the meantime, has replaced the part cut 

 off. If the organism divides into two roughly equal halves we speak of 

 "fission"; "budding" takes place, on the other hand, when the newly de- 

 veloped organism separates from the body of the old one as a compara- 

 tively small branch. Finally, if it is produced from a single cell, which is 

 usually formed in a special organ of the mother body set apart for its pro- 

 duction, we speak of reproduction by "spores." 



In sexual reproduction there are always two, if not independent in- 

 dividuals, then at least physiologically (sexually) different organs, whose 

 products unite: the male, which forms spermatozoa, and the female, in 

 which the ova are formed. In exceptional cases one of the sexes (the male) 

 can be suppressed. We then speak of parthenogenesis, when unfertilized 

 females lay eggs capable of development. Among the higher animals, 

 especially among mammals, parthenogenesis does not occur, although the 

 eggs of birds and mammals often begin to segment without being fertilized. 

 At a time when Darwin was still considering his theory, Hofmeister dis- 

 covered that even the so-called sexless Cryptogams reproduce themselves 



* Reprinted from The History of Biological Theories by Emanuel RadI, translated 

 by E. J. Hatfield, with the permission of The Clarendon Press, Oxford, 1930. 



