THE CRAYFISH AND ARTHROPODS IN GENERAL 229 



blastic. Four stages in the evolution of animals are represented 

 in the groups just mentioned (i) the single cell, (2) a 

 ball of cells, (3) a two-layered sac, and (4) a three-layered 

 organism. 



Early in the past century it was noticed that these stages 

 correspond to the early stages in the embryology of the 

 Metazoa; in other words, that the development of the 

 individual recapitulates the stages in the evolution of the race, 

 or ontogeny recapitulates phylogeny. These stages contrasted 

 appear as follows : 



Phylogenetic Stage Ontogenetic Stage 



(1) single-celled animal egg cell 



(2) ball of cells blastula. 



(3) two-layered sac gastrula. 



(4) triploblastic animal three-layered embryo. 



Later other zoologists became interested in the recapitulation 

 theory, and enlarged upon it. Of these Fritz Muller and Ernst 

 Haeckel are especial- 

 ly worthy of men- 

 tion. The latter 

 expressed the facts Xj 

 as he saw them in &' . 

 his " fundamental 

 law of biogenesis." 

 The ancestor of the 

 many-celled animals FlG - I2 4- Larva of lobster in Af 3-5/5 stage. (From 



was conceived by 



him as a two-layered sac something like a gastrula, which he 

 called a Gastrcea. The Ccelenterates were considered to be 

 gastraea slightly modified. 



Fritz Muller derived strong arguments in favor of biogenesis 

 from a study of certain Crustacea belonging to the Malacostraca. 

 Many members of this group do not emerge from the egg so 



