138 Darwin, and after Darwin. 
pushed inwards until they come into contact with 
those at the opposite pole of the ovum. Consequently, 
instead of a hollow sphere of cells, the ovum now 
becomes an open sac, the walls of which are composed 
Fic. 42.—Gastrulation. A, Gastrula of a Zoophyte ( Castrophysema). 
(After Hackel.) B, Gastrula ofa Worm (Sagi//a). (After Kowalevsky.) 
C, Gastrula of an Echinoderm (Uvraster). (After A. Agassiz.) D, 
Gastrula of an Arthropod (Waupiius). (After Hackel.) E, Gastrula 
of a Mollusk (Lemnews). (After Rabl.) F, Gastrula of a Vertebrate 
(Amphioxus). (After Kowalevsky.) In all, d, indicates the intestinal 
cavity; 0. the primitive mouth; s, the cleavage-cavity; z, the endo- 
derm, or intestinal layer; ¢, the ectoderm or skin-layer. 
of a double layer of cells (C). The ovum is now what 
has been called a gastrula; and it is of importance to 
observe that probably all the Metazoa pass through 
