The Questions of Developmental Biology

stage, the remaining cells divide
normally.
However, instead of producing a normal trochophore
(snail) larva, they produce an incomplete larva, wholly lacking
its mesodermal organs muscles, mouth, shell gland,* and foot.
Moreover, Wilson demonstrated that the same type of abnormal embryo can be produced
by removing the D blastomere from the 4-cell embryo. Wilson concluded that the polar lobe
cytoplasm contains the mesodermal determinants, and that these determinants give the D
blastomere its mesoderm-forming capacity. Wilson also showed that the localization of the
mesodermal determinants is established shortly after fertilization, thereby demonstrating that a
specific cytoplasmic region of the egg, destined for inclusion in the D blastomere, contains
whatever factors are necessary for the special cleavage rhythms of the D blastomere and for the
differentiation of the mesoderm.
The morphogenetic determinants sequestered within the polar lobe are probably located
in the cytoskeleton or cortex, not in the diffusible cytoplasm of the embryo. Evidence for this
localization came from the studies of A. C. Clement (1968). When he separated the animal
hemisphere of the egg of the snail Ilyanassa obsoleta from the vegetal hemisphere, the animal
hemisphere formed ectodermal organs that resembled those formed by lobeless embryos. Clement
then took embryos that had begun resorbing their second polar lobe and placed them into gelatin
slabs. He centrifuged the embedded embryos, forcing the fluid, yolky cytoplasm from the vegetal
part of the cell into the animal hemisphere. By centrifuging these embryos in a second, viscous
medium, he caused the separation of the animal and vegetal hemispheres. The animal halves from
such centrifuged embryos did not develop any more mesodermal or endodermal structures than
those of uncentrifuged eggs. Thus, the determinants of the polar lobe were not transferred to the
animal hemisphere in the fluid contents of the vegetal hemisphere. Van den Biggelaar (1977)
obtained similar results when he removed the cytoplasm from the polar lobe with a micropipette.
Cytoplasm from other regions of the cell flowed into the polar lobe, replacing the portion that he
had removed. The subsequent development of these embryos was normal. In addition, when he
added the soluble polar lobe cytoplasm to the B blastomere, duplications of structures were not
seen (Verdonk and Cather 1983). Therefore, the diffusible part of the cytoplasm does not contain
the morphogenetic determinants. They probably reside in the nonfluid cortical cytoplasm or on
the cytoskeleton.