196 . 



direction and combination of movement in the cells, a dorsiventrality 

 will be induced with the formation of an ingestive aperture on the 

 ventral surface. This is illustated by Arcella in monocytic differentia- 

 tion and by Medusae in polycytic (Fig. 1 a) . 



Hence in the transition between the two, the first stage from the 

 diffuse condition will be a division of labour between the upper and 

 lower cells, the ingestive cells forming the lower half and the locomo- 

 tive cells the upper i^. 



The cells will no longer be polymorphic, the division of labour in 

 time being replaced by division in space and some cells will be inge- 

 stive troughout life, others locomotory throughout life. 



If, as above stated, we assume the metazoan ancestor to be a pela- 

 gic organism, then it is evident that without assuming the survival of 

 accidentally beneficial variations, a stimulus to heterogeneity, and to 

 a condensation of the area of ingestion is found in the dissimilarity 

 in environment between the upper and lower cells. This dissimilarty 

 is most pronounced in the factor of light. It has been shewn ^^ that the 

 direct effect of increased light, within certain limits, upon monocytic 

 organisms, is that of increased activity, so that the greater illumination 

 of the upper surface cells would cause increased locomotory activity 

 in those cells. 



A different reactivity to light conditions might well explain the 

 differentiation of upper and lower surfaces. 



With the dawn of the polycytic individuality combined action 

 between the locomotory cells is possible, and the result is combined 

 action between the ingestive cells. Larger particles are brought round 

 to the ingestive area, and these are enveloped, not by one cell but by 

 the combined act of several cells, which also makes polycytic digestion 

 possible. The final result of this line of evolution must inevitably be 

 the formation of a polycytic ingestive aperture (mouth) and a polycytic 

 digestive sac (Fig. 2). Monocytic ingestion will still take place, but 

 the seat of actual immigration of the ingestive cells will be transferred 

 to the hypoblast layer, whilst the archenteric cavity becomes the seat 

 of polycytic digestion, and under the same evolutional laws a fresh 

 area of polycytic ingestion, the stomadaeum, is formed. This theory 

 of the evolution of the diploblastic from the monoblastic resembles in 

 some particulars that suggested by Mets chnik off ^^, but the point 

 to be especially emphasised here is that monocytic ingestion in the 

 lowest Metazoa involves the mechanical movement of single cells to 



*3 See remarks by B alf o ur (Comp. Emb. Vol. I. p. 149.) on Amphiblastula larva. 

 "Zopf, Encykl. der Naturwiss. Abth. I. Liefg. 1884. 

 »5E. Metschnikoff, Q. J. M. So. Jan. 1884 (Translation). 



