Moss - What genes cant do - 2003, страница 6

Neither C1 norC2 would be construed as either necessary expressions of T nor exhaustive of the developmental trajectories latent in T. Rather C1 and C2should be construed as historically contingent differentiations of T established under certain conditions of existence. Likewise C1 and C2 eachgive rise to further contingent developmental specifications, i.e., O1through O3 and O4 and O5, respectively, which represent specializationin the developmental pathway that focuses and delimits the stock ofKeime und Anlagen which define the possibility-space for these respective orders. From order to family, from family to genus, and from genusto species—each step represents a certain focusing, a certain commitmenttoward the specialized expression of the potential resident at the levelof the previous node, with the resulting diminution of the magnitudeof the resulting possibility-space latent in the developing organism. Asdescribed by von Baer’s laws of development above, earlier stages ofdevelopment are shared by larger and larger numbers of organisms.

Theearlier the stage of development, the less committed to a certain contingent direction of specialization and thus the larger the stock of theoriginal Keime und Anlagen or potential possibility-space. Developingorganisms, for von Baer, never pass through the adult stages of lessadvanced organisms, they merely pass through stages of developmentthat are common to other members of their order, class, family, and soon.

The more specialized an organism becomes the more it progressivelydeparts from other organisms during the course of its development.The Organizing Germ of Nineteenth-Century BiologyVon Baer’s model found its most immediate expression in the elaboration of a descriptive developmental morphology (Entwickelungsgeschichte).

By following the progressive differentiation andspecialization of the forms that the Keime und Anlagen of a type manifest across species, embryologists obtained the means of distinguishingtrue homology from mere analogy. In light of von Baer’s laws, the16Chapter 1“embryological method” became the key to a rigorous methodology,albeit a descriptive one, for discerning authentic phylogenetic relationships.

However, unlike the recapitulationist approach which gainedvogue later in the nineteenth century under the rubric of Ernst Haeckel’s“biogenetic laws,” von Baer’s model did not rely on any form ofintuitionism and did not claim to see the lower adult organism in theembryological pathway of the higher organism. Even more tangibly,developmental morphology provided a comparative method for clarifying some of the more intricate and puzzling aspects of vertebrate, including human, embryology. The rapid success of this approach spoke wellof its heuristic power. Johannes Müller’s classic monograph on theembryology of the vertebrate urogenital system, for example, providesan excellent case in point.Urogenital embryology was complicated by the appearance of apparently vestigial organs and the formation of organs from secondaryassociation of parts which appeared first independently to serve otherfunctions, that is, by the developmental appropriation of degeneratingfetal organs.

As of 1830 the accepted view was that the Wolffian ductwas the common source of both the kidneys and all internal sexualorgans. Müller’s departure from this view was guided by a teleomechanistic perspective. Müller reconstructed the developmental path of theurogenital structures from less complex and less specialized to morecomplex and more specialized by examining a series of vertebrate species,including frogs, salamanders, lizards, snakes, turtles, birds, sheep, goats,cows, dogs, and humans.He was guided by the view that the organs of all of these species arisefrom the same original stock of potential with the more specialized formspassing through the more simple ones during the course of their development.

A closely related assumption was that if one was to claim acausal relation between an earlier and a later structure, one must do soon the basis of the observation of a sequence of structural transformations which confirm the material continuity between the earlier and laterstructures (Lenoir 1982).Demonstrating the separation of the Wolffian body from the kidneys,which develop late in salamanders and at a palpable remove from theGenesis of the Gene17Wolffian body, and demonstrating the absence of material continuitybetween the Wolffian body and the nascent kidney in chicks, mammals,and humans, Müller, employing an analogical manner of reasoningcombined with structural analysis, established that the Wolffian body,or mesonephros, serves as a fetal kidney, but was not the source of themature kidney.

He then established that as the Wolffian body degenerates it is appropriated by the developing male genital system. However,while the Wolffian body is present in male and female vertebrates in earlyembryonic life, careful analysis under the watchword of materialcontinuity, demonstrated that the Wolffian body almost completelydisintegrates in female development.

Where male internal organs—i.e.,the epididymis, vas deferens, and seminal vesicle—are appropriated fromthe degenerating Wolffian body, female internal organs—i.e., the uterus,cervix, and fallopian tubes—are derived from another duct, thepreviously unidentified paramesonephron, henceforth known as theMüllerian duct.Research in developmental morphology, guided by its teleomechanistheuristic, proved to be productive on a number of counts. “A centralunsolved problem in this tradition, however, was that of deciding wherethese Keime und Anlagen reside and in what their nature specificallyconsist of” (Lenoir 1982). An important step in taking the understanding of Keime und Anlagen in the direction of further physical specification was achieved by von Baer in his demonstration of the mammalianovum.

In his celebrated 1827 paper, von Baer began to disclose theprocess of mammalian ovulation through conceiving of the ovum withinits (Graffian) follicle as a kind of egg within an egg, depicting the processof ovum formation as a microcosm of development as a whole. Von Baer,in Kantian fashion, did not believe that either the ovum was prior to thefollicle or that the follicle was prior to the ovum, but rather that theyare the cause and effect of each other. He saw as parallel processes theformation of a smooth membrane on the surface of the follicle and theformation of a smooth membrane on the surface of the ovum.

He identified the nucleus of the ovum not as a material particle but as a centerof formative activity which was responsible for the formation of thegranulosa layer (Lenoir 1982). While this inner core of formative18Chapter 1activity was under the influence of the whole, the process of development itself was construed as largely centrifugal, moving outwards fromcenter to periphery.Modern cell theory proper began in Johannes Müller’s laboratory inthe 1830s with Schleiden’s work on plant cells. Schwann, also a memberof the Müller laboratory, quickly extended Schleiden’s theory of the cellular basis of plants to animals in the course of three papers he publishedin 1838 (Rather 1978).

Although the mood, especially among theyounger generation of investigators, was shifting away from talk ofBildungstrieb or Lebenskraaft and toward a more reductively materialistic language, the new cell theory was both highly influenced by, andquickly incorporated into, the teleomechanist research programs of vonBaer and J.

Müller.In adapting Schleiden’s theory of the plant cell to that of the animalworld, Schwann had followed the pattern established by von Baer. Likethe ovum and the follicle, the nucleus of the cell was perceived as a cellwithin a cell. The idea that morphogenesis proceeded from center toperiphery introduced by von Baer became a dogma of development forSchwann (Lenoir 1982).

The full potential for organismic developmentwas deemed to reside within the cell. Cells, according to Schwann, differalong a continuum as greater or lesser realizations of their potential toform a blastoderm. If living tissue is built of cells and cells are each arepository of the entire Keime und Anlagen of the type, then the questions concerning both normal development and pathological manifestations become situated in a new framework. In the words of Schwann“the individual cells so operate together in a manner unknown to us toproduce a harmonious whole.” (Weiss 1940).While the teleomechanist program enjoyed significant gains withthe discovery of the mammalian ovum and the elaboration of cell theory,these findings would lead to a conceptual crossroads by the lasttwo decades of the nineteenth century.

In 1855, Rudolph Virchow, aformer Müller student and the founding father of cytopathology, proclaimed that all cells are derived only from prior cells: omnis cellulaee cellula. Virchow’s dicta expressed the holism of the teleomechanistlegacy (which could not countenance organized life arising de novo) andyet also introduced the lineaments of a new form of reductionism.

AreGenesis of the Gene19complex organisms no more than the result of second-order interactionsof autonomous cells? Is the ovum then merely a cell like any other cell,or does it also stand in a special relation to the future organism as someform of organizational forebear in a manner in which all its cellularprogeny do not? This distinction was nicely expressed in retrospect byPaul Weiss (1940), whose own work sought to bring together both pointsof view:We may say that the cell theory is correct: the egg is a cell and it gives rise to allthe successive cell generations which contribute to the organism. But the organismic theory is likewise correct: The egg is also an organism, and it passes itsorganization on continuously to the germ and the body into which it graduallytransforms.