12S GENERAL TREXCirLES OE ZOOLOGY 



oxygen, hydrogen, nitrogen, and sulphur have been brought together to produce 

 livmg substance. Tliis process is called spjiilaiicaKS i;fiuTalioii. It' the carbon, 

 oxygen, nitrogen, etc., \vhich are now combined in a stable manner in organisms 

 were formerly unstable, the conditions for the origin of compounds, through 

 whose mder combination life would be possible, may have been more favor.ible. 

 Thus Ihc hvftothcsis of //;<■ first origin of life througli spoilt Jin-ons generation is 

 carried to a logiejl posl:(ljte. 



II. Gi-.xi:r.\tiox by P.vrents, or Tocogony. 



We deal here only with those methods of reproduction which 

 have actually been observed, i.e., generation hv parents. These methods 

 fall mainh- into two great groups, asexual and sexual generation, moiiogoiiy 

 and aiiipliigony, to which may be added a third group, a combiiiation of 

 the two. 



(7. Asexual Reprodiift ion. Monogoiiy. 



Monogony Defined. — The chief characteristic of asexual reproduction 

 is the fact that only a single organism is necessary. But since, in certain 

 modes of sexual reproduction (liermaphroditism, parthenogenesis), 

 this also holds true, further explanation is necessary. Asexual reproduc- 

 tion must be a result of the growth of the organism, w liich has the jK-culiar- 

 ity that it is not growth for an existing individual, but leads to the forma- 

 tion of new individuals. It is noteworthy in this conncctioi\ that many 

 animals can reproduce asexually before they have reached tire normal 

 size (budding in embryo and larval pohzoa and tunicales). This growth 

 may be general and result in an equal growth of all parts; or it nla^• be 

 local and conse(|uenlly lead to the formation of an outgrowth in the 

 region of greatest increase. In the lirst case division t;ilves place, in the 

 latter budding. 



Division. — In the case of division {ef. ilgs. 120, 123, iso) an animal 

 separates into two or more e(iuivalent parts, so that it is not possil)le to 

 distinguish the mother and the daughter animal, for the original animal 

 has completely tlisap]K-arcd in the young generation. The division 

 is commonly a transverse one, the plane of division being perpendicular 

 to the long axis of the animal; less common is longitudinal division, 

 rarest is obli([ue. 



Budding. — In luulding (lig. 03), the products are unecpial. One 

 animal maintains the identity of the mother, while the bud, the out- 

 growth CiYused by local increase, ap])ears as a new formation, as the 

 daughter individual. \'et the difference between tlivision and budding is 

 bridged by intermediate conditions. 



