Cher Belin ....
I would suggest you consider rewriting the sentence as
".....were detected as early as the twentieth day of
gestation in rat embryos before any nerve endings were
observed in the pineal gland."
This is because one assumes more than one rat embryo was
studied so it would be proper to pluralize "embryo" and
this means more than one nerve ending is referred to as
not having developed in the pineal gland. I assume the
object of this sentence which you omitted is also plural and
and that this study ocurred in the past, making the
word "were" apprproiate as a transitive verb (past of etre).