96 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 



numerous experiments, have rejected, even for the latter animals, 

 the idea of spontaneous generation, which, however, still finds in 

 Ponchet* a prominent and zealous supporter. 



Biogenesis, as opposed to abiogenesis, or spontaneous generation, 

 must be regarded as the usual and normal form of reproduction. 

 Fundamentally it is nothing else than a growth of the organism 

 beyond the sphere of its own individuality, and can be always reduced 

 to a separation of a part of the body, which develops into an indi- 

 vidual resembling the parent organism. Nevertheless the nature 

 and method of this process differ extraordinarily ; and various kinds 

 of reproduction can be distinguished, viz., fission, budding (spore- 

 formation), sexual reproduction.-^ 



Reproduction by fission, which, with that by budding and spore- 

 formation, is included under the term monogenous asexual reproduc- 

 tion, is found widely scattered in the lowest animals, and is also of 

 special importance for the reproduction of the cell. It consists 

 simply of a division of the organism into two parts by means of a 

 constriction which gradually becomes deeper, and eventually leads to 

 the separation of the whole body of the organism into two individuals 

 of the same kind. If the division remains permanently incomplete, 

 and its products do not completely sepa.rate from each other, con- 

 pound colonies of animals arise. The number of individuals in such 

 colonies increases by a continuation of the process of incomplete and 

 often dichotomous division of the newly-formed individuals (Yorti- 

 cella, Polyp stocks). The di vision may take place in various direc- 

 tions longitudinal, transverse, or diagonal. 



Budding differs from fission by a precedent disproportionate 

 and asymmetrical . growth of the body, giving rise to a structure 

 not absolutely necessary to the parent organism which is developed 

 to a new individual, and by a process of constriction and division 

 becomes independent. If the buds remain permanently attached 

 to the parent, we have here also the conditions necessary for the 

 formation of a colony (Polyp colonies). Sometimes the budding 

 takes place at various parts of the outer surface of the body, 

 irregularly or obeying definite laws (Ascidians, Polyps) ; sometimes 

 it is localised to a definite part of the body, separated off as a Germ- 

 stock (Salpa, stolo prolifer). The cell-layers distinguished as germinal 



* Pouchet, " Nouvelles experiences sur la generation spontanee et la resist- 

 ance vitale," Paris, 1864. 



f Of. R. Leuckart's article, " Zeugung " in R. Wagner's " Handworterbuch 

 der Physiologic." 



