A coeloblastula is that type of blastula which has a fluid-filled cavity. Since the blastocyst of mammals has a fluid-filled cavity too, can it be called a coeloblastula? My book however mentions the two as different, and it gives the frog as an example of animals having the coeloblastula and mammals as an example of having the blastocyst.
This is a bit tricky.
The blastocoel in e.g. amphibians separates ectoderm from endoderm and prevents cells of the blastocoel roof from inductive influence of vegetal cells. If you cut out the marginal zone and squish the roof onto vegetal cells, you will end up with mesoderm only:
Blastocoel is not homologous to blastocystic cavity. One could consider a tiny space between epiblast and hypoblast during gastrulation as a homologue of blastocoel, this however does not appear in mice. Blastocystic cavity forms at 32-cell stage thanks to Na+/K+ pumps as well as aquaporins. The inner cell mass is a mixture of cells expressing Gata6 and Nanog, which will form hypoblast and epiblast respectively.
But worry not, many people still call that cavity a blastocoel, in fact, I've seen it happen on Wikipedia. Anyway, I'd still recommend using separate terms for coeloblastula and blastocyst due to the non-homologous nature of blastocystic cavity and the blastocoel.
- Gilbert's Developmental Biology
- Ćwiczenia z Biologii Rozwoju Zwierząt [Marek Maleszewski et al., University of Warsaw, Polish language publication]