Common Bauplan for Animal Development

In 1830, a series of eight public debates were held at the Academie Royale des Sciences in Paris. The two opponents, George Cuvier (1769-1832) and Ftienne Geoffroy Saint-Hilaire (1772-1844), were prominent and internationally renowned scientists. Both had made major contributions in many areas of natural history, including comparative anatomy and paleontology. Cuvier divided the animal kingdom into four completely separate branches or embranchements: vertebrates, articulates (largely arthropods and annelids), mollusks (which at the time meant all other soft, bilaterally symmetrical invertebrates), and radiates (echinoderms, cnidar-ians, and various other groups). According to Cuvier, there was no affinity whatsoever between the four embranchements. Any similarities between organisms were due to common functions, not to common ancestry. Function determines form; form does not determine function. Thus, even within these divisions, he allowed structural similarity to result solely from the same functional demands.

Geoffroy, by contrast, insisted that function was always dependent on structure and by no means sufficed to determine structure. What counted were the interconnections between parts; structures in different organisms were the same if their parts were connected to one another in the same pattern. Eventually Geoffroy developed the doctrine of unity of composition, applicable at least within each class of animals. Each animal is formed from a structural blueprint based on a common plan, and although animal structure is modified extensively because of functional requirements, the modification is constrained by the unity of composition (which later came to be known as the basic Bauplan). This doctrine of Geoffroy's came to be known as philosophical anatomy and was founded on analogy between structures (homology in modern terminology). Geoffroy's main criterion for determining true analogies was the connectivity between structures and this could often be better determined from the embryo rather than from the adult. The value of the theory of analogues was that it offered a scientific explanation for differences in structure.

Initially, these ideas related primarily within each class of animals or embranchements, but Geoffroy imagined that the principle could be extended to the animal kingdom as a whole. After having established a common scheme for vertebrates, he extended this principle across the boundaries of Cuvier's four embranchements to articulates. In 1822, Geoffroy published a paper entitled Considerations generates sur la vertebre, in which he proposed that the ventral side of arthropods was analogous to the dorsal side of the vertebrates. This dorsoventral axis inversion hypothesis was based on a dissected crayfish that he had placed upside down

(a)

Figure 2 The dorsoventral inversion hypothesis. a, Geoffroy Saint-Hilaire's dissected lobster. In this dissection, the animal is presented in the orientation opposite to the orientation that it would normally have with respect to the ground. The central nervous system (cns) is at the top and is traversed by the mouth (mo). Below this is the digestive tract with the stomach (s), liver (li), and intestine (in). Below the gut are the heart (he) and main blood vessels (bl). Muscles (mu) flank the CNS. In this orientation, the body plan of the arthropods resembles that of the vertebrate. b, Inverted relationship of the annelid and vertebrate body plans; only the mouth changes position with inversion, making a newopening in the chordate lineage. m, mouth; n, nerve cord; nc, notochord (only in chordates); s, stomodeum (secondary mouth); x, brain. Arrows show direction of blood flow. a, Reprinted by permission from Macmillan Publishers Ltd: Nature (De Robertis, E. M. and Sasai, Y. 1996. A common plan for dorsoventral patterning in Bilateria. Nature 380, 37-40), copyright (1996). b, Modified textbook diagram; see, for example, Romer and Parsons (1977).

Figure 2 The dorsoventral inversion hypothesis. a, Geoffroy Saint-Hilaire's dissected lobster. In this dissection, the animal is presented in the orientation opposite to the orientation that it would normally have with respect to the ground. The central nervous system (cns) is at the top and is traversed by the mouth (mo). Below this is the digestive tract with the stomach (s), liver (li), and intestine (in). Below the gut are the heart (he) and main blood vessels (bl). Muscles (mu) flank the CNS. In this orientation, the body plan of the arthropods resembles that of the vertebrate. b, Inverted relationship of the annelid and vertebrate body plans; only the mouth changes position with inversion, making a newopening in the chordate lineage. m, mouth; n, nerve cord; nc, notochord (only in chordates); s, stomodeum (secondary mouth); x, brain. Arrows show direction of blood flow. a, Reprinted by permission from Macmillan Publishers Ltd: Nature (De Robertis, E. M. and Sasai, Y. 1996. A common plan for dorsoventral patterning in Bilateria. Nature 380, 37-40), copyright (1996). b, Modified textbook diagram; see, for example, Romer and Parsons (1977).

and, as he noted, in this orientation the organization of the main body system of the lobster resembled that of a mammal (see Figure 2). One objection readily raised against such an attempt to link arthropods and vertebrates was that the nervous system in arthropods was nevertheless found on the ventral side, whereas in vertebrates it was located on the dorsal side. Geoffroy's solution to this problem was that the definitions of dorsal and ventral were purely arbitrary, because they were based solely on the orientation of the animal to the sun. If it was assumed that the arthropod walked with its ventral side rather than its dorsal side toward the sun, then all of the organs of the arthropod would have the same topological arrangement as the organs of vertebrates.

As expected, Cuvier rejected such interpretations. For him, animals shared similar basic plans only because they carried out a similar combination of interrelated functions. Because the fundamental plan was completely different in each embranchement, there were no and could be no transitional forms leading from one embranchement to the next. Moreover, no one had ever observed the transformation of one species into another. The differences between the scientific approaches of Geoffroy and Cuvier came to a head when two young naturalists, Meyranx and Laurencet, submitted to the academy a comparison of the anatomy of vertebrates and cepha-lopods (squids, cuttlefish, and octopi), claiming that they were based on the same basic structural plan.

Geoffroy, who was chosen by the academy to review the paper, enthusiastically adopted this claim as proof of his unity of composition shared by all animals. Cuvier could not reconcile this with the results of his careful anatomical research, and in the ensuing debates, he showed convincingly that many of Geoffroy's supposed examples of unity of structure were not accurate; the similarities between vertebrates and cephalopods were contrived and superficial. As an immediate consequence, the results of Meyranx and Laurencet never went to press (for details, see Appel, 1987).

Was this article helpful?

0 0

Post a comment