THE GASTRAEA-THEORY, ETC. 159 



Nikolaus Kleinenherg, in his excellent monograph of 

 Hydra, a work which occupies a prominent ])osition among 

 recent morphological works by the happy union therein 

 of the most accurate objective observation, and clear phi- 

 losophical reflection. Finally, I have myself proved in the 

 biology of the calcareous sponges (1. c, 464), that the two 

 primary germ-lamellse persist throughout life in the sponges 

 in their simplest form, and that the outer animal germ-la- 

 mella simultaneously fulfils the animal functions of sensation 

 and locomotion, the formation of the skeleton and integu- 

 ment, while the inner vegetative germ-layer performs solely 

 the vegetative functions of nourishment and reproduction. I 

 have, at the same time, applied the germ-lamella theory 

 directly to the monophyletic genealogical tree of the animal 

 kingdom, and have thus attempted to supply a firm bio- 

 genetic basis for a Natural system. 



Only the two primary germ-lamellae and the primitive in- 

 testinal cavity which they enclose can be considered as 

 completely homologous, in the strictest sense, throughout the 

 whole animal kingdom (i. e. after excepting the Protozoa, in 

 all Metazoa, from the Sponges to the Vertebrata). The two 

 cell-layers of the gastrula and of the Gastraeada which it 

 repeats, as well as the exoderm and entoderm of the sponges, 

 are, in this strictest sense, doubtless completely homologous 

 v/ith the two primary germ-lamellse in the embryos of the Ver- 

 tebrata, Arthropoda, MoUusca, Echinodermata, and Vermes. 

 The apparent difficulties in the way of this complete homology 

 caused by the formation of a nutritive yelk (and the conse- 

 quent partial grooving incident thereto) in most of the higher 

 animals are easy to set aside and to explain by secondary 

 adaptation. On the other hand, this homology becomes in- 

 complete as soon as the two primary germ-lamellae begin to 

 differentiate and to develop between them a middle cell-layer 

 (mesoderm). The ontogenesis of the plant-like animals and 

 worms plainly teaches us that this middle germ-lamella is 

 constantly derived as a secondary product from one of the two 

 primary germ-lamellae, or perhaps from both simultaneously. 

 One or both of the primary germ-lamellae must, therefore, 

 necessarily undergo a differentiation in the production of the 

 mesoderm, and can consequently be no longer exactly com- 

 pared with the two unaltered and permanent germ-lamellae 

 of the Gastraeada and Sponges (exoderm and entoderm). 

 They must now, like the mesoderm-layer itself, rather be 

 distinguished as secondary germ-layers.^ 



' The primitive homology of the gastrula in all the different animal 

 groups, from the sponges to the Vertebrata, from which we directly infer 



