The Animal Organism and its Germ-Layers 55 



ferentiated bud be called, the germ-layer doctrine is set 

 at nauglit since one laj^er gives rise to both the digestive 

 epithelium and the nerve ganglion. 



As a matter of fact, this statement falls short of reveal- 

 ing the full measure of confusion as regards germ-layers 

 wliich prevails in these animals, since one layer, the outer 

 of the body-wall, contributes to every essential part of the 

 polypide in some species. Thus Calvert shows that in Bug- 

 ula sahaiieri cells are set free into the body cavity from the 

 outer layer at the time that the knob of cells of that layer, 

 which is the foundation of the bud, makes its appearance, 

 and these freed cells assemble to produce, in part at least, 

 the layer on the surface of the knob usually called the meso- 

 derm of the bud. And this observation Romer has confirmed 

 "mit aller bestimmtheit" for another species, Aleyonidium 

 mytile. It seems that the entire polypide may be formed 

 from a single germ-layer, namely the ectoderm. 



The reader should not fail to compare what is here set 

 forth about bryozoan budding with what we learned about 

 ascidian budding to the extent of noticing that whereas in the 

 ascidian we found the inner layer (no matter whether called 

 endoderm or by some other name) producing nearly the entire 

 zooid ; in the bryozoan the outer layer (no matter by what 

 name called) produces in some cases, nearly the entire poly- 

 pide. 



If it be asked whether the principle invoked to explain 

 why the ectoderm of the ascidian bud takes so small a part 

 in producing the future animal (namely, that of greater 

 functional specialization and activity of the ectoderm than 

 of the endodoiTn at the place of origin of the bud,) be also 

 available for explaining the reverse order in layer con- 

 tribution to the bud-produced animal in the bryozoan, no 

 very satisfactory answer is forthcoming from the infor- 

 mation we now possess. Two facts may be adverted to, how- 

 ever, which suggest that the same principle is operative in 



