195 



In the monocytic differentiation, the stage after diffuse ingestion 

 appears to be the formation of a definite ingestive aperture with sto- 

 matic ingestion. 



In the multilaterally symmetrical organism above described one 

 must assume a pelagic habitat, with the environment equalised by a 

 rotation about the centre. It is evident that the rotation about the 

 centre of a sphere must be gradually replaced by rotation about an 

 oral-aboral axis, and this must be caused by a differentiation of the 

 locomotory cells, but the position of the rotatory axis is determined by 



Fig. 1 u. la. 



4 V 



Fig. 1. Transverse section of monoblastic multilateral colony of cells with 

 diffuse monocytic ingestion, digestion and egestion. The dotted area in this and the 

 following figures (la, 2, 2a, 3) represents the area of predominatingly nutritive 

 fluids, whilst the white parts represent the area of predominatingly respiration and 

 excretory fluids. 



Fig. 1 a. Transverse section of transitive form with located ingestive area. 



Fig. 2. Longitudinal section of diploblastic radially .symmetrical colony of 

 cells, with monocytic ingestive, digestive and egestive, and also polycytic processes. 

 (The shaded area represents that of the polycytic nutritive fluid.) 



Fig. 2a. Longitudinal section of transitive form between 2 and 3, with two ru- 

 dimentary coelomic pouches. 



cp coelomic pouch at monocytic stage; e egestive cells; fp food particles ; i in- 

 gestive cells. 



the environment. In such a habit the only different factors in environ- 

 ment are differences in the surroundings of the upper and lower sur- 

 face respectively, so that in further evolution involving also definite 



11* 



