Turbellarian taxonomic database

Abstract / Notes

Early embryology of the Brazilian land planarian, Geoplana notocelis, with a comparative study of
Rhyn-chodemus terrestris, common in Germany. The habits and egg-laying, illustrated by photographs, of the
latter sp. are descr. The egg capsules of Geoplana contain 5-8 embryos and numerous yolk cells. The latter
at first contain several yolk vacuoles but these soon fuse into 1 large vacuole and the yolk cells near each
embryo form a syncytium. The early embryo consists of a group of apparently unarranged blastomeres imbedded
in yolk material which forms a cap over 1 region of the blasto-mere group. The blastomeres elsewhere are
encircled by the yolk syncytium, outside of which occur yolk cells. Up to 200-cells, the blastomeres are all
of the same size; thereafter they become differentiated into large, medium, and small blastomeres. The
embryo consists of 2 central groups of blastomeres, one group of large, the other of small cells, surrounded
by small blastomeres. At about 600 cells, medium-sized cells appear on that side of the large blastomeres,
opposite the small blastomeres. 4 of the large blastomeres differentiate into the lumen cells which draw
apart to bound the lumen of the embryonic pharynx. 4 other large blastomeres form the closing cells of the
lumen. The group of small blastomeres around the lumen cells becomes mesenchymal and forms the greater part
of the pharynx wall, its muscle fibers, etc., while the lumen cells reduce. The other large and the medium
blastomeres become the embryonic intestine. Peripheral small blastomeres unite to produce an epithelial
membrane bounding the embryo. The pharynx is directed towards the yolk cap. When the differentiation of
embryonic pharynx and provisional intestine is completed, the pharnyx by the pumping action of its muscles
ingests the yolk into the intestine, while the closing cells act as a sphincter. The development in general
agrees with that of the paludicolous triclads; Rhynchodemus more nearly resembles the latter than Geoplana
chiefly because of the greater yolk mass in Geoplana.

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