196 Heredity and Environment 



CTENOPHORE TURBELLARIAN 



cct. 



end. 



mes. 



end. 



ECHINODERM 



ASCIDIAN "I 



ASCIDTAN II 



ch, 



FIG. 73. TYPES OF EGG ORGANIZATION IN DIFFERENT PHYLA; cross- 

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

 derm (end} ; clear area, ectoderm (ect). 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; ch., chorda; e.g., cerebral 

 ganglion; v.g., ventral ganglion; pro to., prototroch. 



