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1. The structure of the head in 42 cyclopic, synophthalmic, triophthalmic, anophthalmic and acephalic embryos of Limnaea stagnalis, obtained by the action of lithium, has been studied. 2. Besides the displacement, reduplication, fusion or reduction of the eyes, the following deviations of the normal structure of the head have been obtained: a shortening of the cerebral commissure, with fusion of the right and left cerebral ganglion in the median plane of the head; suppression of the anterior and posterior part of the apical plate, of the head vesicle and velum; and fusion of the lateral tentacle fields into a single median tentacle field. Only in the acephalic embryos, further reductions of head organs (nervous system, statocysts, pharynx) occur. 3. The suppression of the differentiation of the 4 posterior cells of the apical plate occurs with absolute constancy in these embryos. In normal development, these cells are derived from the apical cells, which surround the animal pole of the egg. 4. The results do not lend support to the hypothesis that interactions comparable to the "embryonic induction" known in other groups play a part in the development of the head of Limnaea. 5. They prove, however, that lithium acts on a gradient-field in Limnaea as it does in the echinoids and the amphibia. 6. Recent experiments on the direct effects of lithium on the eggs of Limnaea are discussed.

In: Archives Néerlandaises de Zoologie
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The structure and physico-chemical composition of the egg of Limnaea stagnalis and the changes occurring during the uncleaved stage of the egg were studied by a variety of methods. A. Composition of egg: I. The cytoplasm of the egg consists of 2 parts: ectoplasm and endoplasm,which differ in their staining reactions. Immediately after oviposition, the ectoplasm occupies a sector at the vegetative pole of the egg, the endoplasm the rest of the egg. 2. Three sorts of granules can be distinguished: α-granules (probably mitochondria), β-granules and γ-granules. Moreover, fat droplets and Golgi bodies are present in the egg. 3. The α-granules, which are small, are accumulated especially in the endoplasm. Probably, glycogen, phenolases and peroxydases are bound in some way or other to these granules. 4. The β-granules, of medium size, form a major part of the ectoplasm. They consist of albumen and contain pentosenucleic acids. 5. The γ-granules are coarse, probably albuminous in nature; they lie mostly in the endoplasm. 6. Both fat droplets and Golgi bodies are distributed rather evenly in the cytoplasm, leaving free only the spindle and asters. "Praesubstances" and "Golgi systems" can be distinguished. 7. The hyaloplasm contains pentosenucleic acids in small quantity and, probably, bound sulfhydril components, especially in its central part. 8. The freshly laid egg contains the first maturation spindle in metaphase. The spindle area is free of fat droplets and Golgi bodies, but is characterized by the presence of free glutathion in reduced form. The chromosomes contain thymonucleic acid. B. Changes during uncleaved stage: 9. The course of the maturation divisions and the formation of the polar bodies are described. The egg shows amoeboid movements shortly after the extrusion of either of the polar bodies. This is accompanied with a distinct drop of the tension at the surface. 10. The α- and γ-granules of the endoplasm are attracted by the maturation spindle and asters, forming a halo surrounding the amphiaster. 11. The ectoplasm spreads beneath the egg cortex to the animal side. At first, a gap remains at the animal pole; after the completion of the maturation divisions, the ectoplasm surrounds the whole egg. 12. A fine chorion is formed, which lies inside the first, but outside the second polar body. 13. The spermastcr makes its appearance shortly before the extrusion of the first polar body; during the maturation divisions, it grows slowly. This is accompanied with a gradual rise of the viscosity of the protoplasm. No division of the spermaster with formation of an amphiaster takes place. 14. The sperm-head remains in a subcortical position till shortly after the extrusion of the second polar body; then, it migrates to the spermaster and develops into the male pronucleus. 15. The chromosomes left in the egg after the maturation divisions swell into karyomeres which fuse to the female pro-nucleus. 16. The copulation of the pronuclei takes place immediately beneath the egg cortex at the animal pole. 17. About one hour before cleavage, the animal pole plasm is formed by a local accumulation of substances attracted, probably, by the egg cortex at the animal pole. The centripetal flow of protoplasm in the dilating maturation aster may aid in this localisation process. Many α-granules are transported by it to the animal pole plasm. 18. From the time of oviposition till first cleavage the egg swells considerably, probably by the intake of water. This is accompanied with a decrease in density. At the same time, the γ-granules of the egg give rise to the formation of vacuoles, by an attraction of water from the neighbourhood. Eventually, the egg protoplasm has a vacuolated appearance throughout, with the exception of the animal pole plasm. 19. The viscosity, which is high one hour before cleavage, has a minimum 30 minutes later, then it rises again. The tension at the surface reaches a minimum immediately before the beginning of cleavage.

In: Archives Néerlandaises de Zoologie
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Abstract

1. La distribution des substances dans les oeufs de Limnaea stagnalis, centrifugés dans les dernières vingt minutes avant le début de la segmentation, était étudiée. 2. La segmentation des oeufs centrifugés montrait quelques anomalies. Spécialement, la zone centripète semble mettre obstacle à la formation des sillons. 3. L'oeuf centrifugé montre une stratification en 3 couches: une zone centripète à protoplasme alveolaire; une couche intermédiaire à granules-α; et une zone centrifuge à protoplasme dense contenant les granules-β. Le plasme polaire animal des oeufs normaux n'est pas visible dans les oeufs centrifugés. 4. Dans les premières 2 heures après la centrifugation, les granules-α, amassés dans la couche intermédiaire, se dispersent dans l'oeuf, enfin ils s'accumulent au pôle animal, comme dans les oeufs normaux. 5. Les granules-β, au contraire, demeurent dans la position qu'ils occupent après la centrifugation. La ligne de démarcation entre la partie centrifuge vitelline et la partie centripète sans vitellus reste assez nette. Cette distribution inégale des granules-β n'empêche pas le développement normal jusqu'au stade gastrula ou trochophore jeune. 6. Au stade morula avancé, dans les cellules de la moitié centripète une nouvelle catégorie de granules parait, qui ressemblent aux granules-β dans leur coloration. La formation de ces granules indique un transport de substances protéiques en forme soluble à travers les parois des cellules. 7. Au stade blastula, la coloration des granules protéiques change de l'acidophilie à la basophilie.

In: Archives Néerlandaises de Zoologie
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Abstract

I. The development of Limnaea stagnalis from the first cleavage till the trochophore stage has been studied with various cytochemical methods. 2. The uncleaved egg is surrounded by a vitelline membrane. At cleavage this membrane is carried inward with the cleavage furrows; it forms a partition wall separating the blastomeres. The cleavage cavity arises by a splitting of this wall. Apparently, the vitelline membrane of Limnaea is a layer of living protoplasm forming the outer layer of the egg cortex. 3. The cleavage cavity, which is formed by the coalescence of several lenticular spaces between the blastomeres, widens by the secretion of fluid into this cavity by special secretion cones on the adjacent sides of the blastomeres. The fluid is expulsed to the exterior at regular intervals. In this way, the cleavage cavity plays an important part in the water regulation of the egg. 4. Prior to the 3d cleavage, the subcortical plasm of the egg, which formed a subcortical layer of uniform thickness at earlier stages, concentrates at the animal side and fuses with the animal pole plasm and the perinuclear protoplasm to a common mass of dense protoplasm; the rest of the egg consists of vacuolar protoplasm. Both substances are distributed unequally over the cells at further cleavage; the dense protoplasm, forming the ectoplasm of all cells, is most abundant in the animal blastomeres and decreases in a vegetative direction, whereas the vacuolar protoplasm, which forms the endoplasm, is most abundant at the vegetative side. The ectoplasm is rich in α-granules (mitochondria) and β-granules; the endoplasm in γ-granules and fat droplets. 5. During cleavage and gastrulation, the nuclei have a constant position at the boundary between ectoplasm and endoplasm. The nucleoli show an intense activity; intranucleolar vacuoles are formed, which are extruded, apparently, into the nucleoplasm. An extrusion of entire nucleoli into the cytoplasm is indicated. The nucleoli are rich in ribonucleic acids, sulfhydril compounds, iron and benzidine peroxidase. The cytoplasmic ribonucleic acids and glutathione are, in many cells, most concentrated in the neighbourhood of the nuclear membrane. These facts point to an active role of the nucleus, and especially the nucleolus, in cell metabolism. 6. During later cleavage stages, a transformation of the proteid yolk occurs; the β-granules are dissolved in the cytoplasm, the y-granules increase in number. At the same time, albumen is ingested by the cells from the surrounding egg capsule fluid, and laid down in numerous albumen vacuoles in the ectoplasm. Probably, the nucleus plays a part in these transformation processes ; the same holds true of the Golgi bodies. During further development, the γ-granules decrease in size; ultimately, they disappear altogether. The ingestion of albumen is restricted more and more to the albumen cells of the gut. Both extracellular and intracellular digestion of albumen can be observed. 7. Probably, no direct continuity exists between the Golgi bodies of the uncleaved egg and those found in the cells of the embryo. 8. In the course of development, there is a marked increase of the thymonucleic acid contents of the nuclei. 9. Particular granules, rich in ribonucleic acid, accumulate at the central end of the macromeres, where they fuse into single dark bodies; these bodies are transmitted into the cells of the 4th quartette at the next division; at later stages, they disappear. 10. Glycogen is accumulated in the "central plasm" at the 24-cell stage; at later stages, especially the cells 4a-4c and their descendants are rich in glycogen. 11. Four equatorial groups of thickened ringlets are formed on cells of the 2d quartette during cleavage. 12. With basic vital dyes, in early stages a weak granular staining of the yolk occurs. At later stages, the basic dyes are accumulated in the albumen vacuoles of the cells which may be considered to represent a "vacuome" in the sense of PARAT. Nile blue hydrochloride stains, furthermore, the yolk granules and mitochondria; brilliant cresyl violet gives a diffuse staining of the cytoplasm. Methylene blue and Janusgreen do not penetrate into intact cells. 13. Probably, there exists a relation between the composition of the cells and their further development; especially, the proportion between ecto- and endoplasm seems to be important for the fate of the cells. 14. At the late gastrula stage, the velum and head vesicle show an elective indophenol oxidase reaction; at still later stages, the reaction is especially localized in the shell anlage and mantle fold and in special subepidermal cells of the mesenchyme. 15. At the late gastrula stage, a strong elective benzidine peroxidase reaction of 2 pairs of cells at the lateral angles of the embryo, representing, probably, the descendants of the anterior trochoblasts, occurs; the velum and head vesicle react more weakly. At the late trochophore stage, the gut shows also a rather strong reaction; still later, it is localized in the mantle fold, the shell, and particular mesenchyme cells in the interior of the embryo. 16. The histological differentiation is accompanied with the appearence of considerable differences in chemical composition between the cells. This leads in each of the 3 germ layers to the formation of 2 types of cells: I. large cells, without cell division, with albumen vacuoles, many Golgi bodies, mitochondria and fat droplets, rich in glycogen and iron, but poor in thymo- and ribonucleic acid; they form the larval organs (ciliary cells, albumen cells, protonephrideum); 2. small, dividing cells, without albumen vacuoles, poor in Golgi substance, mitochondria, fat, glycogen and iron, but rich in thymo- and ribonucleic acids; they have to build up the body of the adult snail. These two cell types can be regarded as representatives of 2 different directions of cell life.

In: Archives Néerlandaises de Zoologie
Author:

Abstract

I. La distribution des substances dans les oeufs de Limnaea stagnalis, centrifugés dans les dernières vingt minutes avant le début de la segmentation, était étudiée. 2. La segmentation des oeufs centrifugés montrait quelques anomalies. Spécialement, la zone centripète semble mettre obstacle à la formation des sillons. 3. L'oeuf centrifugé montre une stratification en 3 couches: une zone centripète à protoplasme alveolaire; une couche intermédiaire à granules-; et une zone centrifuge à protoplasme dense contenant les granules-β. Le plasme polaire animal des oeufs normaux n'est pas visible dans les oeufs centrifugés. 4. Dans les premières 2 heures après la centrifugation, les granules-α, amassés dans la couche intermédiaire, se dispersent dans l'oeuf, enfin ils s'accumulent au pôle animal, comme dans les oeufs normaux. 5. Les granules-β, au contraire, demeurent dans la position qu'ils occupent après la centrifugation. La ligne de démarcation entre la partie centrifuge vitelline et la partie centripète sans vitellus reste assez nette. Cette distribution inégale des granules-β n'empêche pas le développement normal jusqu'au stade gastrula ou trochophore jeune. 6. Au stade morula avancé, dans les cellules de la moitié centripète une nouvelle catégorie de granules parait, qui ressemblent aux granules-β dans leur coloration. La formation de ces granules indique un transport de substances protéiques en forme soluble à travers les parois des cellules. 7. Au stade blastula, la coloration des granules protéiques change de l'acidophilie à la basophilie.

In: Archives Néerlandaises de Zoologie
In: Bijdragen tot de Dierkunde