198 



THE EVOLUTION OF MAN* 



that of the cells which constitute the outer, uninverted part 

 (the future exoclerm). When the process is completed, the 

 histological differences in the cells of the two primary 

 germ-layers are usually very strongly marked (Fig. 30). 

 The small, bright-coloured cells of the exoderm {e) are 

 clearly distinguishable from the larger^ darker cells of the 

 entoderm (i). 



Fig. 30. — Cells from the two primary germ- 

 layers of a Mammal (from the two strata of 

 the germ-membrane) : i, the larger, darker 

 cells of the inner stratmn, the vegetative 

 germ-lay er, or entoderm; e,the small, brighter- 

 colom-ed cells of the outer stratum, the animal 

 germ-layer, or exoderm. 



At present we have only con- 

 sidered that form of egg-cleavage, of 

 germ-layer and gastrulation, which 

 on many and important grounds we are justified in regard- 

 ing as the original, j^rimary, and palingenetic form. We call 

 this the primordial, or original, form of egg- cleavage ; and 

 the Gastrula, resulting from this, we call the Bell-gastrula 

 {Arckigastrula). In a form exactly similar to that of our 

 Coral (Moiioxenia, Fig. 22), we meet with this Bell-gastrula 

 in the lowest Plant-animals, in the Gastrophysema (Fig. 23), 

 also in the simplest Chalk Sponges (Olynthus, Fig. 29), 

 in many Medusae and Hydra-polyps ; in low Worms of dif- 

 ferent classes (Sagitta, Fig. 24 ; Ascidia, Plate X. Fig. 1-4) ; 

 again, in many Star-animals {Eckinoderma, Fig. 25) ; in 

 low Articulated-animals (A^ihropoda, Fig. 26), and Soft- 

 bodied Animals (Mollusca, Fig. 27) ; lastly, in the lowest 

 Vertebrate (Amj^hioxus, Fig. 28; Plate X. Fig. 7-10). 



