DIRECT DEVELOPMENT AND METAMORPHOSIS. 



119 



germinal layers throughout the whole Metazoa ; the one being traced 

 bu-k to the ectoderm and the other to the endoderm of the hypothe- 

 tical Gastnea ; 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 Yertebrata 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 ; FlG m ._ Larval stages of the Frog (after Ecker) 



when, ill Other Words, embryo some time before hatching-, with wart-like gill 

 , , , . -I papillae on the visceral arches. I, Larva some time 



tne embryo IS born in after hatching, with external branchite. c, Older larva, 

 an immature condition with norn y beak and small branchial clefts beneath 



the integumentary operculum, with internal branchia? 

 and at a relatively low N, nasal pit; S, sucker; E, branchise; A, eye; II:, 



stage of organization, horny teeth. 



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 adult 



