DilUXT DJSVELOPilEKT AND METAMORPHOSIS. 



119 



germinal layers throughout the whole Metazoa ; the one being traced 

 back to the ectoderm and the other to the endoderm of the hypothe- 

 tical Gastrtea ; while for the middle layer, which is only secondarily 

 developed from one or both of the primary layers, only an incomplete 

 homology was claimed. It cannot, however, be said that this theory, 

 which is essentially an extension of the Baer-Remak theory of the 

 germinal layers from the Vertebrata to the whole group of Metazoa, 

 with its pretentious and hasty speculation has created a basis for 

 comparative embryology ; such a basis can only be obtained as the 

 result of comprehensive investigations. 



DIRECT DEVELOPMENT AND METAMORPHOSIS. 



The more complete the agreement between the just born young and 

 the adult sexual animal, so much the greater, especially in the higher 

 animals, will be the du 

 ration of the embryonic 

 development and the 

 more complicated the 

 developmental processes 

 of the embryo. The 

 post-embryonic develop- 

 ment will, in this case, 

 be confined to simple 

 processes of growth and 

 perfection of the sexual 

 organs. When, how- 

 ever, embryonic life has, 

 relatively to the height 

 of the organization, a 

 quick and simple course ; 

 when, in other words, 

 the embryo is born in 

 an immature condition 

 and at a relatively low 

 stage of organization, 

 the post-embryonic development will be more complicated, and 

 the young animal, in addition to its increase in size, will present 

 various processes of transformation and change of form. In such 

 cases, the just hatched young, as opposed to the adult animal, is 

 called a Larva, and develops gradually to the form of the acini*-- 



FIG. 111. Larval stages of the Frog (after Ecker). a, 

 embryo some time before hatching', with wart- like gill 

 papillic on the visceral arches, b, Larva some time 

 after hatching, with external branchiae, c, Older larva, 

 with horny beak and small branchial clefts beneath 

 the integumentary operculum, with internal branchiae ; 

 If, nasal pit; -S, sucker; K, branchire ; A, eye; Hz, 

 horny teeth. 



