Clonal organization of the central nervous system of the frog. II. Clones stemming from individual blastomeres of the 32- and 64-cell stages.

Horseradish peroxidase injected into individual blastomeres of 32- and 64-cell embryos of Xenopus laevis was identified in cells of the central nervous system (CNS) at larval stages 31 to 39. The CNS received contributions from 24 blastomeres of the 32-cell stage and 38 blastomeres of the 64-cell stage. The region of CNS in which all of the labeled descendants of a single blastomere were dispersed is called a clonal domain. Mingling of labeled and unlabeled cells always occurred in a clonal domain, but boundaries were seen between such a labeled region and completely unlabeled regions. These boundaries occurred at various places in the CNS but were most frequently seen in the transverse plane at the level of the isthmus between mesencephalon and rhombencephalon and in the horizontal plane between dorsal and ventral regions of the CNS. When the maternal clonal domain was partitioned between the descendants of the daughter cells, the partitioning occurred only at one or both of those boundaries. The constant relationship between the position of each of the initially labeled blastomeres and the final spatial distribution of the labeled descendants in the CNS provided a detailed fate map of the main regions of the CNS in the 32- and 64-cell embryo.