318 THE EVOLUTION OF THE META20A 



the early ontogeny taking into consideration the physiological 

 viewpoint. On the basis of everything we now know about 

 the very variable ontogenetic morphogenies it appears as 

 improbable that the first, and thus the oldest, steps in the 

 ontogenies— which had evolved regardless of the phyletic 

 antiquity of total divisions of cells, with few exceptions that 

 can be explained— could possibly represent a recapitulation 

 of the oldest state in the evolution of the Eumetazoa. It 

 seems a priori much more probable that physiological fac- 

 tors, above all the greatly increased need of oxygen as a 

 consequence of a very intensive metabolism, had caused the 

 cellularization to be adopted in the early ontogenies. The orga- 

 nism has thus endeavoured to evolve large surfaces for each 

 active karyon for the sake of an increased gaseous interchange 

 (the intake of oxygen, and the delivery of the carbon dioxide). 

 This has caused the earliest cellularization which took place 

 ad hoc. This trend had led occasionally to a temporary segre- 

 gation, to really anarchic states of blastomeres even if the 

 whole as such had not been lost. The state of the fully devel- 

 oped animal, the plasmodial state, has been recovered after 

 this period of "Storm and Stress," and it can therefore be 

 rightly considered as a primitive state. 



Thus the initially reversible cellularization began in all 

 probability in the earliest ontogenetic stages with cleavage 

 as its very special form. It was much later that true cellula- 

 rization combined with the formation of tissue, had evolved; 

 it remained permanently without losing its ability to return 

 under special conditions into the plasmodial state. Such a state 

 is called a syncytium. Actual cellularization did not appear 

 immediately and generally, but rather by degrees; this is proved 

 by conditions that can be observed in Turbellaria. First it took 

 place in the skin layer (the ectodermal epidermis), later in the 

 intestinal wall (the entodermal intestinal epithelium), and 

 finally in the middle layer where it has usually not been 

 complete (the mesoderm s. 1. or mesohyl, usually divided into 

 the mesenchyme and the coelothelium). 



