Appears this is a new solver, better known to the Groaniad : I'll attempt to cut him some slack.
I have 22 cells, and 11 of those are double-checked by across and down clues. The others, gradually, I have become certain of. And this is my biggest criticism of the puzzle, that having for all intents and purposes solved it, still quite a lot of time must be spent checking the detail.
And I'm still not sure what shape to draw through the cells. Looking at the first line of the themed verse, you'd think that the essence would be reproduction. The shape vaguely resembles a heart, or perhaps the letters SR, or 2 chromosomes in metaphase, or 2 spermatazoans, or perhaps just the 2 numerals of the year in question...I don't know, and don't care any more...
And I think that there is some ambiguity in the entry for 30D, though I plumped for G.