402 THE EVOLUTION OF THE METAZOA 



regards the conditions of the perigastrocoele. The fact must 

 be taken into account that the destiny of the perigastrocoele 

 had been closely connected with the destiny of the mesohyl 

 (mesoderm). We have already repeatedly emphasized that in 

 all probability the whole mesohyl— which is also a substratum 

 of the perigastrocoele— of the Eumetazoa had been inherited 

 by the Eumetazoa from their infusorian ancestors. Viewed 

 from this standpoint we must reject the suggestion which 

 tries to derive the mesohyl from the ectoderm (Ecterocoelia) 

 or from the entoderm (Enterocoelia). Because of the mono- 

 cellular initial state of each ontogeny (the fertilized egg cell), 

 and because of the cellularization which takes place during 

 the ontogenies, it became unavoidable for the ontogenies to 

 develop gradually into a definitive three -layered state. This 

 process can be understood quite generally as a recapitulation 

 of the old adult state inasmuch as there is in these ontogenies 

 first a monocellular state which is followed by a poly cellular 

 state. The same or even greater diversity that we can observe 

 in the ontogenetic development of the entoderm (of the 

 Anlage of the intestinal tube) can also be found in the develop- 

 ment of the third body layer, the mesohyl. It is in principle 

 wrong to try to find special recapitulations of the adult states. 

 We cannot therefore accept a systematic division into the 

 Ectocoelia and the Entocoelia. We could speak on an ectero- 

 coelic and an enterocoehc type of development of the peri- 

 gastrocoele if we had worked exclusively within the field 

 of the comparative ontogenetics. Yet we must refuse to accept 

 such a use of the type of development of the perigastrocoele 

 during the ontogenies for taxonomic purposes because we 

 know that among the Ecterocoelia we can find species or 

 even smaller groups (e.g. the Tardigrada) which develop their 

 perigastrocoele in the enterocoelic way. 



Consequently, we must consider a derivation of the peri- 

 gastrocoele sacks from the "lodges" of the Anthozoa as wrong. 

 This is obvious already for the very fact that we consider the 

 Anthozoa as the most primitive Cnidaria which evolved only 



