IGO GENERAL PRINCIPLES OF ZOOLOGY. 



body (flg. 90). But through vcoent iiivestig;itions exceptions to this rule 

 have become known. In polyzoans ami tiuiicates there are undiffei'en- 

 fiated cells which are employed in ca.ses of budding ; these are elements 

 without the characteristics of a definite Ijody layer which, independently 

 of the positioTi they assume in tlie maternal animal, can be employed, 

 according to need, in the buihling up of organs. In the regeneration of 

 lost parts investigations sliow that it is not necessary that the missing 

 structure, in worms and even in vertcljratcs, shouUl be re-formed by the 

 same layer from whicli it originally arose. The lens of Triton ari.ses 

 ontogeneticahy from the epitlielium of the skin. If extirpated, it is regen- 

 erated from the pigmented epithelium of the iris. 



5. Tlie Different Foriii.< (if Se.nial Development. 



Embryonic and Postembryonic Development. — While the 

 occurrences described (fertilization and cleavage of the agg. forma- 

 tion of the germ-hiycrs) are going on the young animals are 

 usually enclosed within a firm protective covering, or even in the 

 maternal sexual apparatus (uterus), and are lience called emliryos. 

 Later stages, even the formation of the most important organs, 

 may occur during embryonic life, as we see in case of the 

 mammals, birds, reptiles, many fishes, worms, and crabs, which, 

 at the end of their embryonic existence, are complete in all their 

 parts, and need only the maturity of the sexual organs, and growth 

 of the body as a whole, in order to reach the climax of tlieir 

 development. On the other hand, there are animals, chiefly 

 aquatic, which, after leaving the egg, undergo important changes, 

 like the cceleuterates, echinoderms, insects, amphibians, etc. The 

 coelenterates, echinoderms, and many worms usually escape from 

 the egg even before the formation of the germ-layers, and, as free- 

 swimming ciliated 'planulffi,' form the germ-layers and organs. 

 Since there is here a more or less extensive post-embryonic develop- 

 ment, it is a misnomer to apply the term 'embryology*' to both 

 stages; it is necessary, rather, to limit the name to the develop- 

 mental 211'ocesses inside the egg, and, on the other hand, to speak 

 generally of the history of the development of the individual, or 

 ontogeny. As the undeveloped animal within its membrane is 

 called an embryo, so the name hirnt is ajiplicablo to the free-living 

 but not comjiletely matured aninuil. 



Direct and Indirect Development — Metamorphosis. — Larval 

 development may be cither direct or indirect. In direct develop- 

 ment, as the term implies, tlie larva ]iursucs the direct way towards 

 tlie sexually mature aiiinuil, the lacking organs l)cing outlined one 

 after another; hence it is coTitiiuuillv Ix'coming more like the 



