Full Idea
It is essential to an organism arising from sexual reproduction that it has its zygote as initial improper part.
Gist of Idea
The zygote is an essential initial part, for a sexually reproduced organism
Source
Peter Simons (Parts [1987], 7.3)
Book Reference
Simons,Peter: 'Parts: a Study in Ontology' [OUP 1987], p.270
A Reaction
It can't be necessary that an organism which appears to be sexually reproduced actually is so (if you don't believe that, read more science fiction). It may well just be analytic that sexual reproduction involves a zygote. Nothing to do with essence.