THE GASTRAEA-THEORY, ETC. 239 



(ontogenetically) apparently occur first in the outer germ- 

 lamelke, and vice versa. I suspect that this is often actually 

 the case in the sexual cells, and that, generally, such an 

 early transport of the cells has played a significant part 

 through the change of place and change from one germ- 

 lamellse to the other becoming constant by inheritance. This 

 transport also possesses great significance for our above-stated 

 view of the original difference of the two muscular layers, 

 and may, for instance, explain much in the early axial 

 concrescence, in the blending of the germ-lamella in the 

 axial cord of the Vertebrata, as well as in their later diver- 

 gence. 



8. The Significance of the Gastraea-Theory for 



THE Theory of Type. 

 If one judges the above-given confirmation of the Gastraea- 

 theory as sufficient, and acknowledges the conclusions drawn 

 therefrom as on the whole right, one will then have arrived 

 at the conviction that as a consequence the so-called 

 type-theory — which to this very time is in general looked 

 on as the profoundest basis for a zoological system — has 

 been abolished, at least so far as its present significance goes, 

 and an essentially different classification of the animal king- 

 dom put in its place. As is known, this highly renowned 

 and highly meritorious theory of types, which in the second 

 decennium of our century two of the most important con- 

 temporary zoologists attained to by diff'erent ways, culmi- 

 nated in the idea that in the animal kingdom many funda- 

 mentally different principal groups are to be discerned; for 

 each of which peculiar " types" there is a quite charac- 

 teristically immanent and persistent " plan of structure." 

 This plan of structure is determined through the peculiar 

 position and connection of the constitutive organs, and is 

 entirely independent of the grade of perfection and develop- 

 ment traversed by the various classes of animals of each type 

 within its sphere. Both George Cuvier, who, by the path of 

 comparative anatomy, and Carl Ernst Baer, who, alone and 

 independently of Cuvier, arrived at this idea by the path of 

 comparative ontogenesis distinguished in the whole animal 

 kingdom but four such types, which Baer, according to the 

 different manner of ontogenesis, characterised in the follow- 

 ing manner : — (1) Radiata, with a radial development (evo- 

 lutio radiata) ; (2) Mollusca, with a contorted development 



{evolutio contorta) ; (3) Articulata, with a symmetrical de- 

 velopment (evolutio gemina) ; (4) Vertebrata, with a double 

 symmetrical development {evolutio bigemina). Cuvier, as 



