The development of freshwater Turbellaria has been investigated by few scientists, but the embryonic development of Dendrocoelum lacteum was described in detail by Fuliński (1914, 1916). An experimental analysis of the embryonic development of Dendrocoelum lacteum and Planaria torva has been conducted by Seilern-Aspang (1957, 1958). By placing the contents of a cocoon of Planaria torva on a cover-slide one can observe the course of early development. Certain conditions of culture give rise to deformations of the embryonic rudiments, which in turn leads to polyembryony (Seilern-Aspang, 1951b). The phenomenon of polyembryony also occurs in Procerodes lobata (Seilern-Aspang, (1957a). In our investigations normal polyembryony in Dendrocoelum lacteum has been studied in embryonic development not disturbed by experimental factors.

Dendrocoelum lacteum was raised on glass vessels at 20 – 25° C. The animals were fed on Daphnia and pieces of earthworm. One hundred and ten cocoons were fixed on Zenker’s and Serra’s fluids and 5μ sections of them were stained by panoptic method of Pappenheim.

During embryonic development, when the germ consists of several bastomeres, the vitelline cells form a syncytium which differs essentially from the remaining contents of the cocoon. In the syncytium we can easily distinguish two parts; an inner, central one which is more homogeneous and contains loosely scattered blastomeres, and an outer highly vacuolized part containing the nuclei of the vitelline cells. Our investigations (Kościelski, 1964) have shown that the inner part is rich in RNA and mitochondria, whereas numerous greater or small particles of glycogen, and drops of fat, are found in the outer part. The shape of the syncytium is usually spherical (Plate, Fig. A) but sometimes ellipsoidal. In one of the cocoons investigated most of syncytia were ellipsoidal in shape and they were much larger than spherical syncytia (cf. Plate, Figs. B and A). Such elongated syncytia have also been found in other cocoons. They are characterized by a relatively small number of blastomeres widely scattered all over the inner part of the syncytium (Plate, Fig. B). The elongate syncytia can divide. Fission of two germs in one of the cocoons investigated is shown in the Plate, Figs. C and D. In a dividing germ the blastomeres form two agglomerations, each surrounded by a separate inner syncytial mass. The fission proceeds centrally from the outer syncytial layer, dividing an ellipsoidal germ into two spherical embryonic rudiments.

Besides division of the elongated syncytium into two equivalent spherical syncytia, one sometimes encounters a division which does not lead to the formation of two germs. In this case the blastomeres are concentrated in one part, while the separating second one consists solely of the outer syncytial layer (Plate, Figs. E and F).

It appears from the investigations of Seilern-Aspang (1958) that during the early embryonic development of Turbellaria, the ovum and the blastomeres excrete into the surrounding contents of the cocoon some substances (Gruppier-ungstoff, Syncytiumstoff) which are of great importance in morphogenesis. The ‘grouping substance’ causes the vitelline cells to concentrate around the ovum, and then, owing to a ‘syncytial factor’ produced by the blastomeres, they dissolve to form a syncytium. The further differentiation of the blastomeres is continued within it. In Seilern-Aspang’s opinion, the factors determining the development of the germ are partially transferred from the embryonic cells to their surroundings. Thus important elements of development are present not only in the embryonic cells but also in the surrounding trophic mass. A germ, according to Seilern-Aspang, is constituted not only by blastomeres but also by a certain region, the so-called embryonic area (Embryonalfeld), in which both the excreted substances are operating, blastomeres being only a part of this area. To each stage of development there corresponds a particular size of embryonic area. An area lengthening to a size above that corresponding to its ‘maturity’ will split into two secondary areas (as observed in Procerodes lobata, Seilern-Aspang, 1957a). The embryonic area tends to keep its spherical shape. If the embryonic rudiments are flattened on cover-slides, thus disturbing the ratio of the three axes of the germ, it will split into several daughter germs in which the ratio of the axes is approximately normal (Seilern-Aspang, 1957b). Despite the difference in the manner in which the daughter germs of Procerodes lobata and Planaria torva are formed, polyembryony in both cases results from the change in size of the embryonic area, whose size is characteristic and determined for a given stage of development.

Our investigations of the development of Dendrocoelum lacteum confirm the results of Seilern-Aspang obtained from experiments with Planaria torva. Polyembryony in Dendrocoelum lacteum should also be considered as a process related to regulation of the embryonic area. The ellipsoidal area whose shape does not correspond to the given stage of development divides into two daughter areas in which the ratio of three axes approaches the normal one. Elongated, oval, embryonic areas are frequently encountered in the course of embryonal development in Dendrocoeulum lacteum. Since division of such areas seems to proceed very fast, this process is hardly perceived by an observer. So far, we do not know the factors which give rise to the growth and lengthening of embryonic areas. Since the number of germs in the cocoons in which polyembryony was observed did not differ from the normal, the process of polyembryony cannot be explained by the factors which determine such a development in Procerodes lobata.

The tendency shown by the embryonic areas to assume a spherical shape, is manifested also in a division which does not result in two equivalent areas. An ellipsoidal embryonic area regulates its shape by ‘excreting’ a part of the syncytial mass free of blastomeres. This process being similar to that described by Seilern-Aspang (1951b) in Planaria torva raised on cover-slides, is also the confirmation of his data on the morphogenetic rôle of blastomeres in the formation of the syncytium.

The phenomenon of polyembryony in Turbellaria was first described by Seilern-Aspang (1957a) in the development of Procerodes lobata. In this species the above process occurs as late as in larval stage. It appears that in Dendrocoelum lacteum the regulative processes which lead to polyembryony occur much earlier.

In the early development of Dendrocoelum lacteum one sometimes observes processes leading to polyembryony. These processes are related to changes in the size of the embryonic area which is determined for a given stage of development. An ellipsoidal germ divides into two spherical embryonic rudiments whose shape corresponds to that of a normal germ. Regulative processes can also be observed in division of embryonic area which gives rise to only one daughter germ, the second part of the divided area being composed solely of a vitelline mass free of blastomeres. Regulative abilities observed in early development stages of Dendrocoelum lacteum probably explain the phenomenon of experimentally obtained polyembryony.

La Polyembryonie chez Dendrocoelum lacteum O. F. Müller

Pendant le développement précoce de Dendrocoelum lacteum, on observe parfois des processus qui conduisent à la polyembryonie. Ces processus sont en relation avec des changements de grandeur le l’aire embryonnaire, qui est déterminée selon le stade de développement. Un germe elliptique se divise en deux rudiments embryonnaires sphériques, dont la forme correspond à celle du germe normale. Des processus régulateurs peuvent aussi être observés dans la division de l’aire embryonnaire, qui produit un seul germe fille, la deuxième partie de l’aire divisée ne contenant qu’une masse vitelline sans blastomères. Les capacités régulatrices observées pendant les stades initiaux du développement de Dendrocoelum lacteum expliquent probablement le phénomène de la polyembyonie expérimentale.

Fuliński
,
B.
(
1914
).
Die Entwicklungsgeschichte von Dendrocoelum lacteum Oerst. 1 Teil; Die erste Entwicklungsphase von Ei bis zur Embryonalpharynxbildung
.
Bull. Acad. Sci., Cracovie, 147-190
.
Fuliński
,
B.
(
1916
).
Die Keimblatterbildung bei Dendrocoelum lacteum Oerst
.
Zool. Anz
.
47
,
380
400
.
Koṡcielski
,
B.
(
1964
).
Cytological and cytochemical investigations on the embryonic development of Dendrocoelum lacteum
.
O. F. Müller, (in press)
.
Seilern-Aspang
,
F.
(
1957a
).
Polyembryonie ais abnorme Entwicklung bei Procerodes lobata O. Schmidt (Turbellaria)
.
Zool. Anz
.
159
, H.9/10, 187-93.
Seilern-Aspang
,
F.
(
1957b
).
Polyembryonie in der Entwicklung von Planaria torva (M. Schultz) auf Deckglaskultur
.
Zool. Anz
.
159
, H.9/10, 193-202.
Seilern-Aspang
,
F.
(
1958
).
Entwicklungsgeschichtliche Studien an Paludicolen Tricladen
.
Arch. EntwMech. Org
.
150
,
425
80
.
Plate

Fig. A. Spherical embryonic area of Dendrocoelum lacteum. Blastomeres seen in inner part of syncytium; the nuclei of vitelline cells are lying in its external part.

Fig. B. Ellipsoidal, elongated embryonic area of Dendrocoelum lacteum. Blastomeres loosely scattered along the long axis of the embryonic area.

Fig. C and D. The division of two different ellipsoidal embryonic areas each into two daughter spherical germs.

Fig. E and F. Two sections of the same dividing embryonic area. Blastomeres are present in only one part of the dividing embryonic area, the second one consists solely of the vitelline mass.

Plate

Fig. A. Spherical embryonic area of Dendrocoelum lacteum. Blastomeres seen in inner part of syncytium; the nuclei of vitelline cells are lying in its external part.

Fig. B. Ellipsoidal, elongated embryonic area of Dendrocoelum lacteum. Blastomeres loosely scattered along the long axis of the embryonic area.

Fig. C and D. The division of two different ellipsoidal embryonic areas each into two daughter spherical germs.

Fig. E and F. Two sections of the same dividing embryonic area. Blastomeres are present in only one part of the dividing embryonic area, the second one consists solely of the vitelline mass.