Reading them, the mind automatically stumbles a bit; is that "9 embryos that are at the 4-cell stage" or "embryos comprised of 94 cells"? Clearly, some level of compromise is required between "all numerals, all the time" and "every number written as a word", but how best to optimize the tradeoff? Luckily, if we are going to keep our writing concise (and we are), but not sacrifice ease of expression (and we are not), we need only follow a few simple guidelines:
1) Numbers less than 10 (1-9) are usually written out, while 10 and above are expressed as numerals.
Therefore, in the example above, one correct way to write it would be "nine four-cell-stage embryos". "But wait!", you say, "I see '4-cell-stage embryos' all the time! In dozens of papers!" Exactly. In your particular field, conventional usage may have trumped standard grammar rules, so that "nine 4-cell-stage embryos" is perfectly acceptable. It's always a good idea to check with the journal you have in mind to see if they have a preference for either style. For the other two examples, we'd have "54 two-hour samples", but I'd rewrite "75 500-individual groups" to "75 groups of 500 individuals each". Yes, we've added text, but we've dramatically increased clarity.
Important Note: Rule #1 may be broken for the sake of consistency. If numbers less than and greater than 10 appear together in a sentence, express them all as numerals, e.g., "We sampled leaves at 4, 6, 10, 25, and 36 days after germination." "From site A and site B, we obtained 8 and 11 samples, respectively."
2) Numbers at the start of a sentence should always be spelled out.
This one is fairly inviolable. The sentence "25 samples were included in this study" would correctly be "Twenty-five samples were included in this study". However, "278 sites were tested for contaminants" is a little trickier. Instead of burdening your reader with "Two hundred seventy-eight sites were tested for contaminants" , try a quick rewrite: "We tested 278 sites for contaminants." The same goes for "4-cell-stage embryos" - if it starts a sentence but you don't want it spelled out completely, rewrite your sentence!
3) Use numerals for measured quantities
"40 mL water", "5 g tissue", "2.5 min"
4) Leave a space between a number and its unit, except for percentages and degrees.
So, we have "70%" and "70°C" (no space), but "70 mL", "70 days", and "70 mg".
There are more, but these four rules should get you through most of the numerical situations you'll encounter in your scientific writing. (See what I did there?) Remember, the rules of grammar exist to increase the clarity and ease of understanding of any given document. Instead of viewing them as archaic, pointless shackles on your personal expression, try envisioning them as useful tools towards your ultimate goal: easy communication with your readers!