Size regulation in the mouse embryo. II. The development of half embryos.

The study describes an analysis of the development of mouse embryos halved at the 2-cell stage by the destruction of one blastomere, in comparison with control embryos of parallel derivation, at 2.5-13.5 days post coitum. The results showed that: (1) half embryos achieve size regulation some time between 7.5 and 10.5 days; (2) there is an indication that by 13.5 days half embryos may have again dropped back significantly in size relative to controls; (3) preregulation half embryos are slightly retarded developmentally, but this does not wholly account for their smaller size: morphogenesis is not size-dependent; (4) early postimplantation half embryos contain a significantly decreased proportion of inner cell mass derivatives and increased proportion of trophectoderm derivatives when compared with controls. A comparison is also made between the up-regulation of half embryos and the down-regulation of aggregate embryos, and it is suggested that size regulation may occur by delaying a change in the normal growth rate.