rise of fish embryology in the nineteenth century, The

American Zoologist, Jun 1997 by Wourms, John P

The consequences of these advances in embryological technology was to trigger a veritable explosion of embryological studies of teleosts based on the microscopic study of sectioned material, e.g., Van Bambeke, 1876; Hoffman, 1881; Henneguy, 1888; Ryder, 1884a; Kowalewsky, 1886; and Wilson, 1891. By the end of the nineteenth century, an excellent descriptive knowledge of teleost development had been achieved. But there was a trade-off. The emphasis on the study of sectioned material detracted from the observation of living embryos, especially cell behavior during early development, a field that was just beginning to be explored by His (vide infra). This trend led to subsequent difficulties in the interpretation of morphogenetic movements and embryo formation (Trinkaus, 1984a; Solnica-Krezel et al., 1995). Even as this trend was established, its advantages and limitations were becoming obvious. Harrison (1901) published his histological study of the differentiation and axonal growth of spinal neurons in trout embryos in which he focused on the patterns of growth of individual nerve cells and their role in establishing neural connections. The constraints of the histological approach led him to develop tissue culture and use it six years later to investigate neuronal outgrowth (Harrison, 1907).

Development of chondrichthyan fishes

At the onset of the nineteenth century, the study of chondrichthyan development was already farther advanced than that of teleosts. However, the very features that had initially favored the study of chondrichthyan fishes, e.g., large eggs and embryos, viviparity, and internal fertilization, subsequently worked against their further study. It was difficult to obtain access to early stages of development in viviparous species and there was no really effective way to maintain egg-laying species in captivity so as to obtain newly laid eggs. The rate of development was slow. The large, meroblastic eggs were opaque, so that only external features of development and those details revealed by microdissections of embryos could be observed. In contrast, the introduction of artificial fertilization made the early stages of teleostean development readily available and the transparency of most teleostean eggs and embryos was well suited for the microscopic examination of external and internal features of development. Accordingly, nineteenth century studies of chondrichthyan development can be divided into two phases. Phase One spanned the period 1800-1860. During this phase, research progressed along previously established lines with an emphasis on reproductive biology, the study of viviparous species and adaptations for viviparity, and the later stages of embryonic development. Phase Two spanned the period 1860-1911. Research in this phase was shaped by the introduction of new embryological and histological microtechniques to the study of chondrichthyans (see previous section on teleosts for detail).

Research of the eighteenth century passed imperceptibly into the nineteenth century with little or no change of agenda. In addition to his magnum opus, Bloch published a set of separate papers in obscure journals, some of which dealt with chondrichthyan reproduction and development. Tilesius von Tilenau (1802) published a monograph on the egg cases of sharks and skates. It is noteworthy because it represents the first attempts to determine the chemical composition of egg cases, now known to be a type of "tanned" protein. In addition, he also reported on the urogenital anatomy and the reproduction and development of sharks and skates. The major significance of Mitchell's (1803) report on the anatomy of early (7-8 cm) pre-implantation shark embryos is that it is one of the earliest embryological papers and probably the first paper on fish embryology published in the United States.