196 



Heredity and Environment 



CTENOPHORE 



eel. 



TURBELLARIAN 



mes. 



ECHINODERM 



ASCIDIAN1 



mes. 



ASCIDtAN II 



ect. 



end. 



Fig. 73. Types of Egg Organization in Different Phyla; cross- 

 hatched area, mesoderm or mesenchyme (mes) ; horizontal lines, endo- 

 derm (end) ; clear area, ectoderm (cct). In the first four figures the 

 pattern of localization is that which is found at the close of the first cleav- 

 age; in Ascidian II the pattern is that which is found at the close of the 

 second cleavage ; in the annelid egg the localization of later stages is 

 projected upon the egg; n.p., neural plate; eh., chorda; e.g., cerehral 

 ganglion; v.g., ventral ganglion; proto., prototroch. 



