THE CHEMICAL AND EXPERIMENTAL EMBRYOLOGY OF LIMNAEA

Abstract

Summary1. A method is described for obtaining egg masses of Limnaea stagnalis at the moment desired, and the nature of the egg‐laying stimulus is analysed.2. The oogenesis of Limnaea has been studied in detail, with special reference to the formation of various egg substances.3. The insemination of the eggs and the formation of the egg mass are described.4. The development of the fertilized egg from oviposition until the trochophore stage has been studied; special attention has been given to the cytochemical changes in the egg and embryo. A distinction is made between the ‘chemo‐differentiation’, consisting of a redistribution of preformed substances during early development, and the ‘chemical differentiation’, accompanying the histological differentiation. The chemical differentiation leads in all three germ layers to a separation into two types of cells, which specialize in different directions and are destined to form larval and adult organs respectively.5. The osmotic properties of the egg have been studied. The recently laid egg is, on an average, isotonic with a 0.093 M solution of non‐electrolytes.6. With the aid of centrifuge experiments, the swelling of protein granules, the viscosity and tension at the surface of the eggs, their changes during the uncleaved stage and the first cleavages, and the influence of lithium and calcium chloride solutions on these physical properties have been studied. Eggs centrifuged during the uncleaved stage develop into normal young snails in a large percentage of cases. The redistribution of the egg substances after centrifuging has been studied. It is concluded from the experiments that this redistribution is caused by factors residing in the relatively unmovable egg cortex. Apparently the proteid yolk does not play an important part in the determination of the cells.7. The normal cleavage of the egg is dependent on the presence of Ca‥‐ions, but these may be replaced by Lï‐ions in optimal concentration. The Ca‥‐ions influence the properties both of the vitelline membrane and of the egg cortex proper.8. By treatment with lithium chloride solutions both exogastrulation and cyclopean malformations may be induced. For both types of malformation specific periods of susceptibility exist. In the embryos of the cyclopean series the differentiation of the most animal cells of the embryo is suppressed. It is concluded that lithium acts in Limnaea on a polar gradient field. Cyclopean malformations can still be induced at the 24‐cell stage; evidently the pattern of determination has not yet been laid down irrevocably at this stage.9. Recent experiments suggest that two groups of factors are involved in the conformation of the polar gradient field: (a) factors governing the evolution of the mitotic apparatus, which may be influenced by hypertonic solutions; (b) factors directing the cytoplasmic movements, which are susceptible to the influence of lithium.10. Sodium sulphocyanate does not influence the pattern of determination in Limnaea. It causes a wine‐red coloration, the nature of which is still obscure.11. Treatment of the eggs with thiourea causes cleavage abnormalities. Cases of irregularities in cleavage, with deviations in the position of the cleavage spindles, suggest that the factors governing the succession of spindle positions in normal development change relatively independent of the other developmental processes.