<span>Comparative embryology is the study which determines the organisms that have the same and almost identical embryo development.
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While, comparative morphology is the scientific study of organisms where their structures, physical entities and elements are identified to be similar or dissimilar. Using only simple basis of empirical observation, a scientist can detect whether two animals are of the same family or not.</span>
It refers to the cost or benefit of an activity by an economic agent for which it does not pay a price.
Fr ex: factory produces goods , it releases harmful smokes n polluts our enviroment.
In this case externality implies the negetive effect.
Hope it helped u!!
