22 THEORY OF EVOLUTION 



It has been found that the cleavage pattern 

 has the same general arrangement in the early 

 stages of flat worms, annelids and molluscs 

 (fig. 12). Obviously these stages have never 



FIG. 12. Cleavage stages of four types of eggs, showing the 

 origin of the mesenchyme cells (stippled) and mesoderm cells 

 (darker); a, Planarian; b, Annelid (Podarke) ; c, Mollusc 

 (Crepidula), c, Mollusc (Unio). 



been adult ancestors, and obviously if their 

 resemblance has any meaning at all, it is that 

 each group has retained the same general plan 



