i-;8 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 



C . 



Fig ^2 -Gastrulaiion. A, Gastrula of a Zoophyte KGastrophysenid) 

 (After HdckeU B, Gastrula of a Worm ,5a^V/a). (After KowakvskyO 

 Toa^rvlia oi ar.' Echinoderm iUraster). (After A Agassjz ) J 

 Gastrula of an Arthropod i^Nauplius. (A^er Hackel.) E, Ga^^-^ 

 of a Mollusk {^LimncEUsX (After Rabl.) f - Gastmla of ^ J^^^^^^^^^^ 

 i^Amphioxus)) i^KiX.. Kowalevsky.) In all. d, md.ca^e the .Ue tinal 

 cavity- the primitive mouth; s, the cleavage-cavity, t, the endo 

 derm, or intestinal layer ; e, the ectoderm or skm-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 



