198 



THE EVOLUTION OF MAN. 



that of the cells which constitute the outer, uninverted part 

 (the future exoderm). 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 stratum, the vegetative 

 germ-layer, or entoderm; e,the small, brighter- 

 coloured 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, primary, 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 

 (Arckigcbstrula). In a form exactly similar to that of our 

 Coral (Monoxenia, 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 Medusas and Hydra-polyps ; in low Worms of dif- 

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

 again, in many Star-animals (Echinoderma, Fig. 25); in 

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

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

 Vertebrate (Amphioxus, Fig. 28; Plate X. Fig. 7-10). 



