In message , Jaques
d'Alltrades writes

I think they sound very like worm 'eggs'.

They don't sound like earthworm cocoons at all. Much more likely
to be arthropod or mollusc eggs or something added by the nursery that
supplied the plant. Looking back at the original description:

Whilst re-potting a miniature rose, I found a hell of a lot of these:
Cocoons are laid one by one as the worm moves through the soil,
not in masses. I've only found occasional ones, never great piles of
them.

3mm in diameter.
A bit small for earthworm cocoons, except for the smaller
species.

Yellow.
Cocoons are usually brown

Hard (brittle) thin shell.
True for cocoons

Perfectly spherical.
Definitely not spherical, they are tapered at each end. The
cocoon is secreted by the clitellum as a collar and when the worm
retreats out of it the ends contract and seal. There is a rather grotty
picture at:
http://www.scarboro.demon.co.uk/garden/earthworms.htm

The vesicles line up and the male part of each worm inseminates the
female part of the other. As part of the process (It's nearly fifty
years since I studied this at school, so memory is a bit hazy) the
clitellum is worked down each worm and the sperm and ova from the female
vesicle are squeezed into the clitellum, which is shed, and its ends
close up, encapsulating the fluids.

Your memory is a bit hazy. Earthworms lie head to tail with the
male pores of one opposite spermathecal pores of the partner (and visa
versa). Sperm is exchanged and they go their own happy ways. Later over
periods weeks or months, each worm secretes a cocoon from the clitellum,
The worm retreats out of the cocoon, depositing into it ova (eggs) and
sperm from the female and spermathecal pores respectively. Fertilization
takes place in the cocoon but, although several ova are deposited only
since worms have been seen to emerge.

Of course, there are some worms that don't bother with meeting
up with another worm and just produce cocoons from which identical
copies of themselves emerge. (Come to think of it I don't think anyone
has checked whether the offspring are completely identical at the DNA
level.)
--
Amynthas
email address ROT13'd