338 Metschmkoff on Germ-Layers. [April 
cheta and Insecta(3). He found in these animals the same 
three layers, and carefully studied their changes. The reju- 
venated germinal-layer theory had now gained a firm basis in 
the domain of the Invertebrates, and comparative embryology 
took a new direction, mainly under the guidance of German and 
Russian investigators. Since the idea of the germinal layers 
was taken from the higher animals, and was then applied to the 
Invertebrates, it was natural that misconceptions should arise, 
owing to such an anti-genealogical method. Some of these mis- 
conceptions have lasted until to-day ; for instance, in cases where 
the determination of the several layers is beset with doubt there 
is often too much stress laid upon purely topographical charac- 
teristics. The Orthonectide and Dicyemide afford two such 
cases. In these animals certain cells, whose function is genera- 
tive, are styled endoderm, merely because they lie beneath the 
external layer. Ed. van Beneden(14, 15) goes so far as to con- 
sider the topographical position as the one guide in determining 
the germinal layers. To quote his own words: “We designate 
as endoderm the layer or mass of cells which is enclosed, what- 
ever be the tissues derived from it.” Haeckel must in this re- 
spect be accredited with having made an important step forwards, 
when he sharply formulated the view, according to which the 
germinal layers, or at least the two chief layers, are to be re- 
garded as primitive organs(16). From this stand-point a structure 
in question could only be called endoderm when it possessed 
several characteristics of this primitive organ, and not when it 
merely agreed with the organ in topographical position. If, for 
example, the enclosed mass of cells in the Orthonectidze were 
digestive in function there would be no doubt that they repre- 
sented an endoderm; but since the cells in question are sexual 
cells, there is very considerable doubt. The main difficulty in 
the determination of the layers is due to the fact that the gene- 
alogy of the germinal layers does not rest on a safe basis, since 
we know nothing of the primitive condition of the Metazoa. To 
