GENERAL EMBRYOLOGY. 193 



the young animals are usually still enclosed within a firm 

 protective covering, or even in the maternal sexual ap- 

 paratus (uterus), and are hence called embryos. Later 

 stages, even the formation of the most important organs, 

 may fall in the time of embryonic life, as we see in 

 case of the mammals, birds, reptiles, many fishes, worms, 

 and crabs, which, upon arriving at the end of their em- 

 bryonic existence, are completed in all their parts, and 

 in addition 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 marine, which, after leaving their egg- 

 membranes, still undergo important changes, like the ccclcn- 

 t crates, ecJiinoderms, insects, ampliibians, etc. The cccle li- 

 terates, ccJiinodcrms, and many worms usually break through 

 the coverings even before the formation of the germ-layers, 

 and, as free-swimming ciliated ^ planulce, " form the germ- 

 layers and organs. Since there is here a more or less 

 extensive post-embryonic development, it is a misnomer 

 to apply the term "embryology' to both stages; it is 

 necessary, rather, to limit the name to the developmental 

 processes inside the egg-membrane, 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 a direct or an indirect 

 one. In the direct development the larva, as the name 

 implies, pursues the direct way towards its goal, the sex- 

 ually mature animal, since the organs which are lacking to 

 it are laid down one after another, and hence it is contin- 

 ually becoming more like the sexually mature animal. 

 The indirect development, on the contrary, makes devia- 

 tions ; organs are formed which later are destroyed, belong- 

 ing only to the larval life, and hence called larval organs. 

 Therefore in the definition of indirect development, or as 



