rise of fish embryology in the nineteenth century, The

American Zoologist, Jun 1997 by Wourms, John P

SYNOPSIS. The nineteenth century was critical for the empirical and conceptual growth of developmental biology. Fishes played a central role in this process. The study of fish development, mainly that of teleosts but also chondrichthyans, can be traced back to classical times. In the nineteenth century, it merged with modern descriptive embryology, continued with the rise of comparative embryology associated with evolutionary studies, and moved into the experimental and physiological analysis of development. Any consideration of fish development must take into account that fishes phylogenetically are the most diverse group of the vertebrates and also the most speciose. These features are reflected in the diversity of their development. The descriptive embryology of fishes is reviewed from Aristotle to the beginning of the nineteenth century. The study of chondrichthyans, especially viviparous species, was characteristic of this period. During the nineteenth century, there was a progressive development of knowledge of the descriptive embryology of teleosts and chondrichthvans. Teleosts came to the fore because artificial fertil

ization ensured a ready supply of material and their transparent eggs were well suited for microscopy. The subsequent development of embryological microtechnique made possible the examination of sectioned material and moved research to a more cellular level. By the end of the century, an in-depth description of development was in place. Interest in the comparative embryology of fishes was stimulated by Haeckel's melding of embryology and evolution and led to a description of development of agnaths, chimaeras, lungfish, and primitive actinopterygian fishes. Experimental and analytical methods of inquiry began to be used at mid-century. The experiments of Ransom on the contractility of egg cytoplasm, Lereboullet's experimental teratology, chemical studies of embryonic nutrition in viviparous fishes, in vitro observation of blastomeres, His's concrescence theory of embryo formation and Kastschenko's and Morgan's testing of it are considered.

INTRODUCTION

It is particularly appropriate to write about the history of fish embryology at this time. History is often said to move in cycles, and this seems to be true of the study of fish development. After a period of prominence during the nineteenth and early twentieth centuries, the study of fish development, except in the hands of a stalwart few such as Oppenheimer and Trinkaus working with Fundulus and Ballard and Devillers working with the trout and other fishes, was overshadowed by developmental studies of other organisms, such as amphibians, the domestic fowl, sea urchins, and assorted invertebrates. For obvious reasons, the study of fish development continued to be pursued by fisheries biologists. With the popularization of the zebrafish Danio rerio as a "model of vertebrate development" and a revival of interest in the relationship between development and evolution, the cycle has come full turn and the study of fish development again has a place in the sun.

My objective is to present an outline of the historical development of the science of fish embryology in the nineteenth century, broadly defined as 1789-1914. My plan is to assess the state of knowledge at the outset of the period, to document the advancement of descriptive embryology of teleosts and elasmobranchs, to consider the rise of comparative embryology of fishes associated with evolutionary studies, and to search out the beginning of experimental, physiological, and biochemical analyses of fish development.

When discussing the development of fishes, it is prudent to recall that fishes are the most diverse group of vertebrates (Fig. 1). They should not be treated as a distinct, relatively homogeneous taxonomic unit. Rather, the term "fishes" applies to a grade of ectothermic, aquatic craniates and vertebrates and not to a distinct taxonomic group (Atz, 1985; Bruton, 1990; Nelson, 1994). Extant fishes are divided into five classes, namely Myxini (hagfish), Cephalaspidomorphi (lampreys), Chondrichthyes (cartilaginous fishes), Sarcopterygii, and Actinopteryg (Nelson, 1994). Chondrichthyes contains two taxa, namely the subclass Elasmobranch (living sharks, rays, and skates) and the Holocephal (ratfishes or chimaeras) that diverged at a relatively early time, i.e., the Devonian-Carboniferous boundary (Compagno, 1990). The class Sarcopterygii (flesh-finned fishes) contains the Actinistia (the living coelacanth), the Dipnoi (lungfishes) and the tetrapod vertebrates. The class Actinopteryg (ray-finned fishes) comprises five groups, namely Cladistia (bichir and reedfish), Chondrostei (sturgeons and paddlefish), Ginglymodi (gars), Halecomorphi (Amia calva, the bowfin), and the Teleostei (teleost fishes) (Lauder and Leim, 1983; Atz, 1985; Nelson, 1994). Thus, the five classes of fishes contain eleven major taxonomic groups with an estimated total of 25,000 species. Each class is as distinct from the others as it is from the four other classes of vertebrates. The theme of eleven major taxonomic groups of fishes that is used in the ensuing presentation to emphasize the broad phylogenetic relationships of fishes and other vertebrates, tends, however, to obscure the extraordinary phylogenetic diversity within the teleosts. Teleostei, a monophyletic group, contains 24,000 species in 38 orders and 126 families compared to the 1,000 species, 19 orders, and 56 families in the ten other groups of fishes and the 25,000 species of tetrapods (Nelson, 1994).