In two or more groups of animals, however much they may differ from each other in structure and habits in their adult condition, if they pass through closely similar embryonic stages, we may feel assured that they are all descended from one parent-form, and are therefore closely related.
Nahiz eta beren heldu-egoeran bata bestetik ezberdinak izan egituran eta azturetan, baldin enbrioi-fase oso antzekoetatik iragaten badira, bi animali talde edo gehiagori buruz ziur egon gaitezke guraso-forma bakarretik datozela eta, beraz, oso hurbilak direla.