192 THE EVOLUTION OF MAN. 



germ-cavity, or cleavage-cavity. The latter decreases in 

 proportion as the former extends. At last the central germ- 

 cavity entirely disappears, while the inner, inverted portion 

 of the germ-membrane, the wall of the groove, attaches its 

 inner surface to the inner surface of the outer, uninverted 

 portion of the germ-membrane. At the same time, the cells 

 of the two parts assume a different form and size ; the inner 

 cells become rounder ; the outer become longer (Fig. 22, /). 

 The germ thus acquires the form of a cup or goblet- 

 shaped body, the wall of which consists of two different 

 cell-layers, while the cavity in its centre grows outward at 

 one end, at the place where the inversion originated. This 

 highly important and interesting germ-form is called the 

 germ-cup or the intestinal larva (Gastrula, Fig. 22, /, in 

 longitudinal section ; A", surface view).^^ 



The Gastrula seems to me the most important and 

 significant germ-form of the animal kingdom. For in all 

 true animals, the Protozoa excepted, the egg-cleavage 

 results either in a genuine, original, palingenetic gastrula 

 (Fig, 22, /, K), or in an equivalent kenogenetic germ- 

 form, which has arisen secondarily out of the earlier form, 

 and which may be referred directly back to that form. 

 It is certainly a most highly interesting and significant fact, 

 that animals of the most diverse tribes, Vertebrates, Soft- 

 bodied Animals (MoUusca), Articulated animals (Arthro- 

 Ijoda), Star-animals {Ecldnodernia) , Worms, and Plant- 

 animals {Zoophyta) develop from one common germ-form. 

 In most striking illustration of this, I place side by side 

 several genuine Gastrula forms, taken from tribes of animals 

 (Fig. 23-28, with the description). 



This extraordinary importance of the Gastrula makes 



