168 PRINCIPLES OF ANIMAL BIOLOGY 



Some investigators still hope that a simple living substance may be syn- 

 thesized under special conchtions of temperature, moisture, and pressure 

 by the combination of the elements carbon, nitrogen, oxygen, hydrogen, 

 phosphorus, iron, etc., usually found in protoplasm. This hope, which 

 has not been widely entertained, has so far been disappointed, and, al- 

 though chemists have been able to synthesize many organic compounds 

 once thought to be incapable of production by other means than the 

 activities of cells, the manafactare of the complex mixture called pro- 

 toplasm is still beyond their reach. 



The modern conception of spontaneous generation differs from the 

 ancient view in that it does not propose to account for the origin of organ- 

 isms of the kind we now know. It therefore has no relation to ideas 

 regarding reproduction, and further discussion of it is omitted. 



Reproduction. — Since no Uving thing can maintain itself for an un- 

 limited period of time the ability of the members of a species to produce 

 other individuals is most important from the standpoint of the welfare 

 of the species. Living organisms provide for increase of numbers of 

 individuals by a variety of reproductive methods which fall into two 

 general categories, namely, asexual reproduction and sexual reproduction. 



Asexual or non-sexual reproduction includes all those methods of 

 reproduction which require but a single parent for the production of 

 offspring and do not involve germ cells. Sexual reproduction as a rule 

 involves two parents and the production of two kinds of germ cells, the 

 eggs and sperms. It is usually brought about by the union of a sperm 

 cell with an egg, or less commonly by the development of the egg with- 

 out union with a sperm. 



Asexual Reproduction: Budding. — A simple and common mode of 

 asexual reproduction is that of budding. The bud starts as a protrusion 

 of protoplasm in a small area of the surface of a protozoan cell or as a 

 localized proliferation of cells in a multicellular animal. The pro- 

 tuberance grows until it assumes the form and perhaps the size of the 

 parent. It usually develops organs similar to those of the parent and 

 either becomes independent of, or remains attached to, the parent. 

 Budding is a rare reproductive process among the Protozoa but is 

 common in certain groups of the metazoa. 



Nearly all sponges bud. Among the Crelenterata, the common 

 Hydra shows the process well and is a good example of an animal whose 

 buds separate from the parent. Other budding members of the same 

 group are Hydractinia (Fig. 63), Obelia, Bougainvillea (Fig. 61) and 

 other Hydrozoa, also the corals, and the Siphonophora (Fig. 64). Some 

 of these have been described in Chapter V. These forms all demonstrate 

 lateral budding or budding from the side of the parent, and some of them 

 produce tree-like colonies by the failure of the buds to separate from the 

 parent. Among the worms the most usual form of budding is terminal 



