The Questions of Developmental Biology

of the embryo, the yellow lipid inclusions migrate with it. This migration forms a yellow
crescent, extending from the vegetal pole to the equator (Figure 8.38B-D); this region will
produce most of the tail muscles of the tunicate larva. The movement of these cytoplasmic
regions depends on microtubules that are generated by the sperm centriole and on the wave of
calcium ions that contracts the animal pole cytoplasm (Sawada and Schatten 1989; Speksnijder et
al. 1990; Roegiers et al. 1995).
Edwin Conklin (1905) took advantage of the differing coloration of these regions of
cytoplasm to follow each of the cells of the tunicate embryo to its fate in the larva (Figure 1.7).
He found that cells receiving clear cytoplasm become ectoderm; those containing yellow
cytoplasm give rise to mesodermal cells; those that incorporate slate-gray inclusions become
endoderm; and light gray cells become the neural tube and notochord. The cytoplasmic regions
are localized bilaterally around the plane of symmetry, so they are bisected by the first cleavage
furrow into the right and left halves of the embryo. The second cleavage causes the prospective
mesoderm to lie in the two posterior cells, while the prospective neural ectoderm and
chordamesoderm (notochord) will be formed from the two anterior cells. The third division
further partitions these cytoplasmic regions such that the mesoderm-forming cells are confined to
the two vegetal posterior blastomeres, and the chordamesoderm cells are likewise restricted to the
two vegetal anterior cells.
Autonomous and conditional specification of tunicate blastomeres
As mentioned in Chapter 3, the autonomous specification of tunicate blastomeres was
one of the first observations in the field of experimental embryology (Chabry 1888). Reverberi
and Minganti (1946) extended this analysis in a series of isolation experiments, and they, too,
observed the self-differentiation of each isolated blastomere and of the remaining embryo. The
results of one of these experiments are shown in see Figure 3.8. When the 8-cell embryo is
separated into its four doublets (the right and left sides being equivalent), mosaic determination is
the rule. The animal posterior pair of blastomeres gives rise to the ectoderm, and the vegetal
posterior pair produces endoderm, mesenchyme, and muscle tissue, just as expected from the fate
map.
From the cell lineage studies of Conklin and others, it was
known that only one pair of blastomeres (posterior vegetal; B4.1) in
the 8-cell embryo is capable of producing tail muscle tissue.
These cells contain the yellow crescent cytoplasm (Figure
8.39). When yellow crescent cytoplasm is transferred from the B4.1
(muscle-forming) blastomere to the b4.2 (ectoderm-forming)
blastomere of an 8-cell tunicate embryo, the ectoderm-forming
blastomere generates muscle cells as well as its normal ectodermal
progeny (Whittaker 1982; Figure 3.10). Moreover, cytoplasm from
the yellow crescent area of the fertilized egg can cause the a4.2
blastomere to express muscle-specific proteins (Nishida 1992a).
Conversely, Tung and colleagues (1977) showed that when larval cell
nuclei are transplanted into enucleated tunicate egg fragments, the
newly formed cells show the structures typical of the egg regions
providing the cytoplasm, not of those cells providing the nuclei. We
can conclude, then, that certain determinants that exist in the
cytoplasm cause the formation of certain tissues. These
morphogenetic determinants appear to work by selectively activating