THE GASTR^EA. 63 



of two cell-strata, which are, in fact, the two primary germ- 

 layers, the animal skin-layer, and the vegetative intestinal 

 layer. 



The ontogenetic origin of the gastrula from the blastula 

 at the present day affords us trustworthy intelligence as to 

 the phylogenetic origin of the Gastraea feom the Planaea. 

 We found that on one side of the globular germ-membrane 

 vesicle a groove-like depression begins, and this inversion 

 (invaginatio) becomes continually deeper (Fig. 171, IT). At 

 last it is so great, that the outer, inverted part of the germ- 

 membrane, or blastoderm, attaches itself closely to the inner, 

 uninverted portion (Fig. 171, /). Now, if guided by this 

 ontogenetic process, we wish to conceive the phylogenetic 

 origin of the Gastrsea in accordance with the fundamental 

 law of Biogeny, we must imagine that the one-layered cell- 

 society of the globular Planaea began, especially at one point 

 of its surface, to absorb nourishment. At the nutritive point 

 on the surface of the ball a groove-like depression was gra- 

 dually formed by natural selection. The groove, which was 

 at first quite shallow, in course of time became continually 

 deeper. The function of nourishing, of absorption of 

 nutriment, and digestion, was soon limited to the cells 

 which lined the groove, while the other cells undertook the 

 function of movement and covering. Thus originated the 

 first division of labour among the originally homogeneous 

 cells of the Plansea. 



The first result of this earliest histological differentia- 

 tion was the distinction of two different kinds of cells ; 

 within the hollow the nutritive cells, without, on the sur- 

 face, the motive or locomotive cells. The distinction of the 

 two primary germ-layers was thus caused. The inner cells 

 38 



