160 GENERAL PRINCIPLES OF ZOOLOGY. 



body (fig. 90). But through recent investigations exceptions to this rule 

 have become known. In polyzoans and tunicates there are undifferen- 

 tiated cells which are employed in cases of budding ; these are elements 

 without the characteristics of a definite body layer which, independently 

 of the position they assume in the maternal animal, can be employed, 

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

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

 structure, in worms and even in vertebrates, should be re-formed by the 

 same layer from which it originally arose. The lens of Triton arises 

 ontogenetically from the epithelium of the skin. If extirpated, it is regen- 

 erated from the pigmented epithelium of the iris. 



5. The Different Forms of Sexual Development. 



Embryonic and Postembryonic Development. While the 

 occurrences described (fertilization and cleavage of the egg, forma- 

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

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

 maternal sexual apparatus (uterus), and are hence called embryos. 

 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 their 

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

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

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

 coelenterates, echiuoderms, and many worms usually escape from 

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

 swimming ciliated 'planulae/ 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 processes 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 larva is applicable to the free-living 

 but not completely matured animal. 



Direct and Indirect Development Metamorphosis. Larval 

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

 ment, as the term implies, the larva pursues the direct way towards 

 the sexually mature animal, the lacking organs being outlined one 

 after another; hence it is continually becoming more like the 



