rise of fish embryology in the nineteenth century, The

American Zoologist, Jun 1997 by Wourms, John P

Three important structures, characteristic of teleostean development, still need to be dealt with, namely the yolk syncytial layer or periblast, the enveloping layer, and Kupffer's vesicle. Although Lereboullet (1862) first described the yolk syncytial layer, it was von Kupffer (1868) who suggested that the cells of the blastoderm not only give rise to the tissue cells of the embryo but also are progenitors of the yolk syncytial layer. Agassiz and Whitman (1884, 1915) proved beyond any doubt that yolk syncytial nuclei are derived from the marginal cells of the blastoderm. The enveloping cell layer is a tightly sealed epithelial monolayer that forms the outermost boundary of the blastoderm. Priority for its discovery is unclear, for descriptions of it under various names appeared more or less simultaneously in a number of independent studies. Kupffer's vesicle is a transient structure that is only found in teleost embryos. It was discovered by von Kupffer (1866), who thought that it might be an allantoic rudiment. It is a small, fluid-filled epithelial sac that appears during the somite stages. Kupffer's vesicle develops mid-ventrally in a region below the tail bud and at the interface between the embryo and yolk mass. Even now, its function, if any, still remains unknown (Brummet and Dumont, 1978).

At this point, a digression to consider the development of viviparous teleosts is in order. Following the pioneering studies of Forchhammer (1819) and Rathke (1833) on Zoarces, knowledge of viviparity in teleosts progressed steadily through the rest of the century. In 1844, Duvernoy described the development of a freshwater poeciliid fish from Surinam. His work was mainly concerned with the later stages of development, but did provide some important information on early development. The ovary is the site of gestation. Fertilization takes place in the follicle where the embryo is invested by an egg envelope. Development to term takes place in the follicle. Duvernoy illustrated for the first time an early stage follicular embryo in its egg envelope. In his Memorias, published 1851-55, the Cuban naturalist Poey was able to extend the number of viviparous species to include all ten species of Cuban poeciliids. Of greater significance were his observations for the first time of sperm storage in these fishes. Unfortunately, he interpreted his observations in terms of parthenogenesis rather than sperm storage and superfetation (Jacobson, 1977). The problem was not satisfactorily resolved until 1908, when Phillipi discovered that in poeciliids, sperm is packaged into spermatozeugmata that are transferred to the male reproductive tract. He also presented evidence for specific structural pathways, which he called "delle," that allow sperm to move from the lumen into close proximity with the egg and fertilize it within the follicle. In 1856, Wyman produced a definitive study on the embryology of the foureyed fish Anableps. It, too, is a fish with follicular gestation. He depicts the elaborate follicular epithelium and the highly vascularized abdominal trophoderm of the embryo. Because of the massive increase in size of the embryo during gestation, he postulated that the developing embryos are nourished by secreted maternal proteins. He extended his speculations to include embryonic nutrition in the surfperch and the electric ray Torpedo.