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Develomental stages

If a zygote, created, we allow to divide,
And is animal, a blastula
(Or ball of cells) is soon espied.
Before it moves on to the gastrula.

The gastrula takes the ball
And puts in it a dent
Which deepens to the other wall,
And punctures there a vent

If the dent's the mouth, then mouth is first
(In Greek that's protostomes:
A group of invertebrates dispersed
Throughout all the biomes.)

I'd speak on invertebrates more, but well,
It's really tricky goin's
When detail forces you to spell

Otherwise chordates, echinoderms
Or hemichordates seek asylum.
Second the mouth - in proper terms
Deuterostomia (a superphylum)

But, in the developmental timing,
One stage in each phylum is singular
(But causes abuse of rhyming)
The phylotypic stage, pharyngula

Though creationists it will rile 'em,
It's the same in every thing-you'll-a
llow into the phylum,
The phylotypic stage, pharyngula

Animals all have a blastula
You can divide them up by gastrula
Pharyngula the phylum ensnares
All according to von Baer's
Principle of developmental creation
By means of increasing specialisation.

Aye, 'tis doggerel, but happy 50'th birthday, PZ Myers!


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