Had a major look into the developing embryos of Lineus sp. and here are the observations:

0. Perfectly round fertilized [light] yellow eggs encased in bundles of 10-20 by a membrane.
1. Some cleaving embryos are observed. Apparently one or two per bundle. I commonly saw 8 cell stage radial/holoblastic embryos.
2. There are undivided embryos and in the first cleavage and some that look like dissociated blastomeres (80% size of original embryos). Otherwise cleavage patterns do not seem regular; embryos have blastomeres of different sizes, apparently asynchronous. For each bundle there is at least one embryo in late cleavage/morula/blastula stage which I assume to be the first embryo to undergo cleavage in the previous day.
3. Each embryo from a bundle in different state. Some weird looking cleavages. A few blastula stages visible and maybe an initial gastrulation.
4. Same as day 2 (and 3?), but: there are not many dividing embryos anymore, most embryos acquired a rough surface (no perfectly round eggs anymore), some blastula stages and some embryos (blastomeres?) apparently fusing.
5. [do]
6. Embryos look more like blastulae of different sizes and shapes. Some seem to be product of fusion of 2 embryos and some smaller ones might be dissociated blastomeres.
7. [do]
8. Embryos are more rounded with a smoother surface which indicates epithelia consolidation of blastula stage (probably began at 6d). I see many gastrulae, at least 1 per bundle (not all bundles are visible), with a clear blastopore on the vegetative pole. Maybe there were present since 7d, check.
9. [do]
10. [do]