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 



FIG. 42. Gastrulation. A, Gastrula of a Zoophyte (Gastrophysema). 

 (After Hackel.) B, Gastrula of a Worm (Sagitta). (After Kownlevsky.) 

 C, Gastrula of an Echinoderm (Uraster). (After A. Agassiz.) D, 

 Gastrula of an Arthropod (Nauplius}. (After Hackel.) E, Gastrula 

 of a Mollusk (Limnaus). (After Rabl.) F, Gastrula of a Vertebrate 

 (Amphioxus). (After Kowalevsky.) In all, d, indicates the intestinal 

 cavity ; o. the primitive mouth ; s, the cleavage-cavity ; i, the endo- 

 derm, or intestinal layer ; e, 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 



