Stages of oocyte development in Cynops pyrrogaster are defined, and changes of annulate lamellae in their fine structure, number, sizes and locations during oogenesis are described. The results show that two different types of annulate lamellae occur during oogenesis. One type differentiates in or at the periphery of vesicle-rich cytoplasm at the early stages of vitellogenesis and increases in number and size. The maximum number of about 40 stacks per median section of oocyte is reached at the stage of complete differentiation of the animal and the vegetal hemispheres. In these growing oocytes, all the stacks show elongate appearances and tetragonal arrangements of annuli as common characteristics. A second type of stacks of annulate lamellae is added anew in full-grown oocytes, increasing the number of stacks per median section of the oocyte to about 90. The new stacks occur in close contact with electron-dense bodies in the cytoplasm and have a massive appearance and hexagonal array of annuli. It is suggested that they appear coincidentally with the onset of oocyte maturation. The possible significance of the observed results is discussed.

Changes in the fine structure, the location and the number of stacks of annulate lamellae during progesterone-induced maturation of oocytes of Xenopus were determined by electron microscopy. In longitudinal sections of full-grown oocytes, about 260 stacks of annulate lamellae were observed with marked concentration in the subcortical layer, particularly in the vegetal hemisphere. After exposure to. progesterone, annulate lamellae increased and accumulated further in the subcortical layer. A significant increase of annulate lamellae around the vegetal side of the germinal vesicle was seen 3 h after progesterone application. In oocytes 6 h after progesterone application, an average of 320 stacks of annulate lamellae were seen in longitudinal sections and more than twothirds of the pore complexes of annulate lamellae were localized in the subcortical layer less than 50 prn from the oocyte surface, the rest being distributed in the deeper ooplasm. At the time of germinal vesicle breakdown, all the annulate lamellae underwent complete decomposition. The results were discussed from the view point of comparative developmental biology.The presence and fine structure of annulate lamellae in amphibian oocytes have been known for a long time, but the roles of these lamellae are unknown (1, 2).Recently, changes in annulate lamellae during maturation of oocytes of the newt were studied by electron microscopy by examination of stacks of annulate lamellae in longitudinal sections of oocytes at various stages of maturation (3). The annulate lamellae were found in three areas of the oocyte during maturation: the cortical layer, the middle area of the vegetal hemisphere and the area near the yolk-free cytoplasm, which is formed at the vegetal side of the germinal vesicle (4, 5, 6).The localizations of annulate lamellae, particularly in the cortex and the yolk-free cytoplasm, coincided with areas which have been discussed in relation to morphogenetic activities (7, 8, see also 9). From the topographical coincidence of locations of annulate lamellae and of presumed morphogenetic determinants and also from the possible presence of RNA on annulate lamellae (1,3) it seems necessary to examine the possibility that the material on annulate lamellae is involved in programming of the eggs for embryonic development, as originally suggested by BALINSKY and DEWS (10). For this the general changes in annulate lamellae in different species must first be examined.Changes of annulate lamellae during maturation have been observed in Xenopus luevis 1

Sizes of yolk platelets were measured in sections of oocytes and embryos in Xenopus. It was found that the average size of the largest group of platelets in cells differed between germ layers of neurulae. It was small (3 to 5 μm) in the ectoderm, medium-sized (5 to 8 µm) in the mesoderm, and large (over 8 μm) in the endoderm. Platelets of these size classes formed layers in egg, the yolk gradient, by the end of oocyte maturation. The yolk gradient contained products of the mitochondrial cloud and a part of the germinal vesicle material at certain positions. The layers of small, medium and large platelets in the egg changed their locations to distribute to the ectoderm, mesoderm and endoderm of neurulae, respectively. The yolk layers in the egg thus represented different prospective fates, and a figure describing the locations of these layers could be regarded as a fate map for the one-cell stage. Most of the marginal blastomeres of embryos at cleavage stages consisted of a few parts with different prospective fates. Results were discussed with reference to available fate maps for cleavage stage embryos.

No abstract

It has been indicated that specification of the dorsal marginal mesoderm of the Cynops gastrula is established by vertical interactions with other layers, which occur during its extended involution. In the present study, when the prospective notochordal area of the early gastrula was almost completely removed together with the dorsal mesoderm-inducing endoderm and most of the bottle cells, the D-less gastrulas still formed the dorsal axis with a well-differentiated notochord; in half of them, where the involution occurred bi-laterally, twin axes were observed. On the other hand, when the wound of a D-less gastrula was repaired by transplanting the ventral marginal zone and ectoderm, the formation of the dorsal axis was inhibited if the involution of the lateral marginal zone was prevented by the transplanted piece. The present study suggests that: (i) cells having dorsal mesoderm-forming potency distribute farther laterally than the fate map; and (ii) the extended involution plays an essential role in the specification of the dorsal marginal mesoderm, especially in notochordal differentiation in normal Cynops embryogenesis.