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 



FlG. 42. Gastrulalion. A, Gastrula of a Zoophyte (Gastrophysemd). 

 ( After HackeL) B, Gastrula of a Worm (Sagittd). (After Kow.ilevsky.) 

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

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

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

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

 cavity ; o. the primitive mouth ; t, the cleavage- cavity ; ', the endo- 

 derm, or intestinal layer ; t, 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 



