This cell might be making a big mistake.  One of the more than 40 chromosomes* is lagging far behind the others as they pull away from the metaphase plate in anaphase, separating duplicate copies from each other at the climactic moment of mitosis.  That’s probably its sister right above it, lagging slightly too.

It’s possible that this chromosome will catch up; it might still be connected by microtubules to the appropriate spindle pole, and eventually get reeled in before the cleavage furrow gets there.  Even if it doesn’t catch up, it’s possible that, by chance, the cleavage furrow will happen to pass by it in such a way that each daughter cell still gets the proper number of chromosomes.  But if this one little parcel gets routed the wrong way, then both cells are probably doomed.

It’s not that they will be missing important genes: even if things go wrong, each daughter cell will have a complete set of genes, because, thanks to fertilization, this zygote is a diploid cell.  As a diploid cell enters mitosis, it duplicates the versions of each chromosome from the mother and the father, so that by this point it has four copies of each gene.  Since the chromosome that’s in danger of getting mis-routed contains just one of those four copies. 

Therefore the worst possible thing that could happen is that one daughter has only one copy of all the genes on this chromosome, and the other daughter has three copies.  But this condition – aneuploidy, in this case a monosomy and a trisomy – is still likely to be very bad news.  In humans, as far as I know, only monosomy of the X chromosome is non-lethal, and that’s because of the dosage compensation mechanism that makes males with one X and females with two X’s more or less equivalent.  Trisomies are bad news too; the most prevalent non-lethal human trisomy is a trisomy of chromosome 21, which leads to Down syndrome.  Having three copies of most other chromosomes is lethal, at least to the whole organism, if not to individual cells.  This fact shows that it’s not just whether or not the organism has a particular gene, but that the dose of the gene matters.  A gene isn’t just a set of instructions, it’s a physical participant in the biosynthesis of whatever gene product it encodes.

A chromosome sorting error in the zygote (or during meiosis) is of course the worst, because every cell in the embryo will be affected by this uncorrectable error.  Errors during later divisions create mosaics.  If one of your cells, right now, made such a mistake, it would found an aneuploid lineage.  Would it make you sick?  Maybe.  To do the arithmetic is a little startling: a sea urchin embryo will conduct 10-12 rounds of mitosis before it hatches and swims off into the sea as a blastula that consists of a few thousand cells.  It will already have conducted on the order of 100,000 trials of its chromosome sorting machinery (40+ chromosomes at each mitosis, times N–1 divisions to create N cells).  And that’s just to make a blastula!

* I’m sorry to admit that I don’t know exactly how many chromosomes a purple urchin has, despite staining many, many of them.  It’s hard to count.  The thing is, even in anaphase when they are highly condensed, some of them are very close to each other, making it difficult to know whether there’s one or two, and there are also several very small ones.  So it would require squashing to count accurately, but there’s definitely forty-something.