The Questions of Developmental Biology

Clement (1962) also analyzed the
further development of the D blastomere
in order to observe the further
appropriation of these determinants. The
development of the D blastomere is
illustrated in Figure 8.27. This
macromere, having received the contents
of the polar lobe, is larger than the other
three. When one removes the D
blastomere or its first or second
macromere derivatives (1D or 2D), one
obtains an incomplete larva, lacking heart, intestine, velum (the ciliated border of the larva), shell
gland, eyes, and foot. When one removes the 3D blastomere (after the division of the 2D cell to
form the 3D and 3d blastomeres), one obtains an almost normal embryo, having eyes, foot,
velum, and some shell gland, but no heart or intestine (Figure 8.33). Therefore, some of the
morphogenetic determinants originally present in the D blastomere must have been apportioned
to the 3d cell. After the 4d cell is given off (by the division of the 3D blastomere), removal of the
D derivative (the 4D cell) produces no qualitative difference in development. In fact, all the
essential determinants for heart and intestine formation are now in the 4d blastomere, and
removal of that cell results in a heartless and gutless larva (Clement 1986). The 4d blastomere is
responsible for forming (at its next division) the two mesentoblasts, the cells that give rise to
both the mesodermal (heart) and endodermal (intestine) organs.
The material in the polar lobe is also responsible for organizing the dorsal-ventral (backbelly)
polarity of the embryo. When polar lobe material is forced to pass into the AB blastomere
as well as into the CD blastomere, twin larvae are formed that are joined at their ventral surfaces
(Figure 8.34; Guerrier et al. 1978; Henry and
Martindale 1987).
Thus, experiments have demonstrated that
the nondiffusible polar lobe cytoplasm is extremely
important in normal mollusc development for a
number of reasons:
It contains the determinants for the proper cleavage
rhythm and cleavage orientation of the D blastomere.
It contains certain determinants (those entering the 4d blastomere and hence leading to the
mesentoblasts) for mesodermal and intestinal differentiation.
It is responsible for permitting the inductive interactions (through the material entering the 3d
blastomere) leading to the formation of the shell gland and eye.
It contains determinants needed for specifying the dorsal-ventral axis of the embryo.
Although the polar lobe is clearly important for normal snail development, we still do not
know the mechanisms of its effects. There appear to be no major differences in mRNA or protein
synthesis between lobed and lobeless embryos (Brandhorst and Newrock 1981; Collier 1983,
1984). One possible clue has been provided by Atkinson (1987), who has observed differentiated
cells of the velum, digestive system, and shell gland within lobeless embryos. Thus, lobeless