STRUCTURAL AND TOPOCHEMICAL CHANGES IN THE EGG CELLS OF LIMNAEA STAGNALIS L. DURING OOGENESIS by L. H. BRETSCHNEIDER AND CHR. P. RAVEN (Zoological Laboratory, Universityof Utrecht) The structural and topochemical relations of the freshly-laid egg of Limnaea, as described by RAVEN (1945), arise gradually during oogen- esis from the much simpler structure of the early oocyte. In order to obtain a better understanding of the structure of the full-grown egg cell, which formed the object of our experimental researches (RAVEN and BRETSCHNEIDER 1942 and further papers, cf. RAVEN I gq.8), an ex- tensive investigation of oogenesis seemed desirable. This investigation had to include the study of various cell compounds by cytochemical methods. In this way, it should be possible to get some insight into the presumable interrelations of these diverse compounds in a cell with a high and very active metabolism. Besides a qualitative study of the origin of the structural and chemical differentiations of the egg also their relations with the growth of the egg cell should be studied quanti- tatively. In order to get comparable results, the same cytochemical reactions have been employed as in our former papers (RAVEN and BRETSCHNEI- DER I gq.2, RAVEN 1945, 1946). The quantitative determinations of cell compounds have been made by means of centrifuge experiments. The snails were centrifuged in toto for 10-15 minutes at a centrifugal pressure of I86o X gravity; immediately afterwards, the gonads were excised and fixed. From the sections, the layers of cell substances strati- fied by centrifugation were drawn on cardboard at a constant magnifi- cation, and the drawings were cut out and weighed. In this way, the relative volumes of the cell compounds were determined ; by comparison with the actual total volume of the egg cell they can be reduced to actual volumes (in ,u3). In most of the diagrams, however, for the sake 2 of simplicity not the actual cell volume, but the product of length and breadth of the largest section of the cell (called "cell-index") has been used as a measure of cell size. Since the measurements have all been made on fixed and sectioned material, the values found are less than those observed by RAVEN (1945) on living eggs, in consequence of shrinking during dehydration and embedding of the gonads. THE ANALYSIS OF OOGENESIS S I. PHASES AND RATE OF DEVELOPMENT During oogenesis various phases of multiplication, growth, different- iation and rest may be distinguished. From the study of young Limnaea's reared from the eggs the duration of these phases, at least as regards the first generation of eggs, could be determined. The characteristics, sequence and duration of these phases have been summarized in fig. I. a. From primary sex cell till oogonium The first phase of development, beginning with the fertilized egg (A) and ending with the oogonia (D) of the recently-hatched young snail, is characterized by a period of intensive division of the egg cell, during which the primary sex cells are formed. This phase lasts 3o days; 15 days are taken up by embryonic development till the formation of the gonadal anlage, whereas the postembryonic development of the latter with the multiplication of the primary sex cells (C) and their transformation into oogonia takes another 15 days. b. Amoeboidphase In the hermaphroditic gonad the egg cells develop, as contrasted with the spermia, between the connective tissue and the epithelium lining the cavity of the ovariotestis. In this retroepithelial position, the egg cells show an intense amoeboid motility (E). They swarm out from a germinal centre to various positions in the acini of the gonad. This phase, which lasts 6 days, is characterized by an initial growth of the early oocytes (cell index increasing from 170 to 1200 a2). Their shape is very variable, and true pseudopodia are present. c. Growth phase, Now the oocyte becomes sessile and is surrounded with nutritive cells apically and laterally, which form a follicle around the egg; only its basal part borders upon the connective tissue (F). Most of the pheno- mena of growth and differentiation described below occur during this .