OOps! This one! -- Sneaking

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Date Posted: 09:20:04 01/10/04 Sat
Author: Me
Subject: OOps! This one!
In reply to: Me 's message, "The Wolf And The Lamb (NT)" on 09:14:18 01/10/04 Sat

as an attempt to answer fundamental embryological questions, and their opposing positions�Wilson's acceptance of the chromosome theory and Morgan's long-standing rejection of it�will be seen in the context of their commitments to certain embryological theories.
And the embryological theories must first be grounded in an even larger controversy over whether biological processes are directed by soluble, diffusible molecules or by morphological structures (2). In fertilization physiology, F. R. Lillie championed the idea that there were cell-surface receptors that bound the sperm and thereby activated the egg. Indeed, Lillie prpopsed that changes in the shape of the receptor after it had bound sperm were important in mediating egg activation (3 and this website--chapter 4). His theories were against those of Jacques Loeb, a fertilization physioologist who championed the effects of soluble factors in activating the egg. His experiments on parthenogenesis (1899, 1916; see this website, chapter 4) had shown that the sperm wasn't even needed, and he would claim (4), "I consider the chief value of the experiments on parthenogenesis to be the fact that they transfer the p[roblem of fertilization from the realm of morphology into the realm of physical chemistry."
In a similar dispute in immunology , the "morphological" Paul Ehrlich postulated that the immune response was based on stereospecific side-chains on the cell surface. He was countered by Svante Arrhenius who believed that antibodies followed the laws of mass action. As we will see, Wilson was to take the "morphological" position, claiming that physical bodies--chromosomes--were the critical determinants of hereditary and developmental phenomena; while Morgan took the "soluble interactions" position, claiming that heredity and development were directed by cytoplasmic factors interacting according to chemical laws (2).
Edmund Beecher Wilson ( 1856-1939), Theodor Boveri (1862-1915), and Thomas Hunt Morgan (1866-1945) were among the first group of embryologists "liberated" by developmental mechanics. Before its metamorphosis, embryology had been studied as an adjunct to evolutionary phylogeny, and both Wilson and Morgan had done excellent studies delineating phylogenetic relationships by cell-lineage analysis. But with the advent of developmental mechanics, many old questions were left behind while new ones took on central importance. Entire methodologies and modes of explanation and evidence were substituted for older ones in what had become a new type of embryology. In the same year that Roux called for "a study of embryonic physiology,''(5) E. B. Wilson celebrated the reemergence of embryology as a new discipline: "So long as the study of embryology was dominated by the so-called biogenetic law, so long as the main motive for investigation was thesearch for phylogenetic relationships and the construction of systems of classlfication, the earlier stages of development were little heeded."(6).
Morgan echoed the call for a "more exact, more profound . . . new embryology,"(7) and Boveri claimed that the day would come when embryology would be a biochemical science (8). As of 1900, none of these researchers felt the need to ground their work in Darwinism, and neither Morgan nor Wilson felt that natural selection was an adequate explanation for the origins of developmental phenomena (9). The value of an explanation had moved from its ability to explain the salient features of evolution�conslancy with variation, recapitulation, analogies and homologies�to the much smaller realm of being able to account for the physiological capabilities of the egg or embryo itself.
Indeed, Wilson and Morgan were very similar in both background and method. Both men received their doctorates from the Johns Hopkins University in the laboratory of William Keith Brooks (Wilson in 1881, Morgan in 1890). Upon completing their respective theses, both went to the Naples Marine Biology Station and became friends of Dohrn and Driesch. In particular, Wilson became a close friend of Boveri. In 1885, Wilson became the first professor of biology at Bryn Mawr, and when he left to go to Columbia University in 189t, Morgan took his place in Pennsylvania. Later, Morgan would join Wilson at Columbia (1904). Their families were close and even were neighbors at Woods Hole during the summers. (So close were their histories that the pycnogonids on which Morgan worked for his thesis were studied twelve years earlier by Wilson, and both of them worked on invertebrate regeneration in addition to their studies of early embryogenesis).

It was precisely in this area of early development that the central problem of developmental mechanics was framed: Which of the two major compartments of the fertilized egg�the nucleus or the cytoplasm�controls heredity and development? This problem did not arise de novo but emerged as the extension of the celllineage question which had hitherto occupied embryologists. Wilson's 1893 lecture at Woods Hole characterized the new studies as being motivated by recent insights into the fertilized egg which might enable researchers to account for suchobservations as the "so-called pre-morphological relations of the segmenting ovum" and the differential partition of egg substances by cleavage (l0). The first conflict between partisans of the nuclear hypothesis and proponents of the cytoplasmic hypothesis involved the localization of preformed hereditary and morphological determinants. That some directing substance or substances had to exist was a necessary assumption, shared even by evolutionary morphologists like William Keith Brooks, the thesis advisor to both Wilson and Morgan. Something must cause the egg of one species to develop differently from that of another species even though the eggs look identical and are in the same environment (11). This observation linked heredity to development, and the identification and localization of such determinants became the central problem for the new embryology. Coleman (12) and Churchill (13) have shown that localization and identification indeed constituted a single problem, since hypotheses postulating a determinant assumed a knowledge of where it acted, and hypotheses concerning the location of embryological determinants eliminated certain candidates for that role. Moreover, Coleman's and Churchill's studies highlight a controversy which would be extremely important for the new developmental physiologists: were the hereditary and developmental agents morphological entities (chromosomes, idioblasts, and so on), as 0. Hertwig and C. Nageli claimed, or were they soluble chemicals reacting in a physiological manner, as postulatedby E. Pfluger and T. L. W. Bischoff? he means sought to identify the determinants was to clarify their location. At first, the cytoplasm of the egg had been proposed as the location of these predetermined substances. W. His hypothesized that anlagen were present within the egg cytoplasm that were destined to form specific bodily parts (l4). This theory of Organbildende Keimbezirke had several important supporters, among them Flemming, van Beneden, and Lankester, the last of whom said of the egg cytoplasm: "Though the substance of a cell may appear homogeneous under the most powerful microscopy, it is quite possible, indeed certain, that it may contain, already formed and individualized, various kinds of physiological molecules.'' (15). C. 0. Whitman also supported this theory, extrapolating from his observations on the worm Clepsine: "While we cannot say that the embryo is predelineated, we can saythat it is predetermined. The "histogenetic sundering" of embryonic elements begins with the cleavage, and every step in the process bears a definite and invariable relation to antecedent and subsequent steps... It is, therefore, not surprising to find certain important histological differentiations and fundamental structural relations anticipated in the early stages of cleavage, and foreshadowed even before cleavage begins. " (16).Soon, however, there evolved another germinal localization hypothesis, placing the site of inheritance in the nucleus. Nageli had proposed that there existed within the egg cell an idioplasm which formed the physical substance of heredity, and which was separate from the nutritive trophoplasma of the egg. Although Nageli did not specify the nucleus as the seat of this idioplasm, new evidence of the importance of the nucleus in protozoa regeneration, and of the fidelity of chromosome number, morphology, and transmission, led Hertwig, Strasburger, Kolliker, and Weismann to conclude independently that "the nucleus contains the physical basis of inheritance, and that chromatin, its essential constituent, is the idioplasm postulated in Nageli's theory."(l7). This line of reasoning was further developed by Roux and Weismann. Roux, who certainly championed a physiological approach to embryology, exempted the idioplasm from epigenesis: "While it is true that the normal operations of development are essentially physiological problems, we must, nevertheless, not lose sight of the cardinal fact that the organization of the idioplasm, which is at the bottom of every such reaction, is an inheritance from the past". (18).Roux pictured the nucleus as a heterogeneous array of substances, each having a position on the chromatin fibers. Division, then, could be equivalent, each substance being exactly duplicated and passed to the progeny of the cell, or it could be quantitative. In the latter case, theoriginal array of qualities is unequally apportioned between the daughter cells, thereby causing a differentiation to occur. Weismann extended this idea to its logical conclusion: that in a fully differentiated cell, only one of the many original determinants remains present, and that only in germ cells are all the determinants retained.This hypothesis of quantitative nuclear division was severely upset by the experiments of Driesch, who showed, first by completely severing the blastomeres of sea urchin eggs and later by changing their cleavage orientations by pressure, that the nucleus of a sea urchin blastomere destined originally to become ectoderm can retain the ability to become endoderm or even to generate the entire embryo itself. Driesch believed that "the results of ontogenesis come about by chemical phases" (19) and held that the nucleus and cytoplasm interact to produce the harmony of development. The fate of a blastomere was determined by its position rather than by the loss of chromatin material: "Insofar as it carries a nucleus, every cell, during ontogenesis. carries the totality of all primordia; insofar as it contains a specific cytoplasmic cell body, it is specifically enabled by this to respond to specific effects only... When nuclear material is activated, then, under its guidance, the cytoplasm of the cell that had first influenced the nucleus is in turn itself changed, and thus the basis is established for a new elementary process, which itself is not only a result but also a cause." (20).
ven though Driesch's hypothesis differed greatly from the one proposed by Roux, the nucleus was still deemed the site of the primordia.But direct proof for the nuclear control over development was still lacking, and critics of this hypothesis were able to point out that it was primarily an interpretation based on the role of the nucleus in fertilization and the constancy of nuclear organization within species. In 1889, in a paper appearing in a German journal, even Boveri acknowledged this limitation (2l). He then appended the results of what he believed to be the definitive experiments proving the determinative role of the nucleus in development . Boveri fertilized presumed nonnucleated egg fragments of Sphaerechinus granularis with the sperm of another sea urchin, Echinus microtuberculatus. The skeletal axes of their pluteus larvae differ considerably, so that the observation of the skeleton of the larvae produced from such a mating should allow one to infer whether the nucleus (from the sperm) or the cytoplasm (from the enucleate egg fragment) controlled the developing structure. As indicated by the paper's title, "An Organism Produced Sexually without the Characteristics of the Mother," Boveri reported that the hybrid larvae had the skeletal axes of the male parent only; hence, the nucleus controlled development. This work was translated from the original German I, Aystyr, sneak Okohdako and her bonded Asojhuten from Land of Lace to Selma Oasis into English for The American Naturalist by Thomas Hunt Morgan (22). Morgan notes that this article should be translated so that English-reading audience can judge it, because it dealt with a problem of major importance (23) but whose chances of being correct are by no means assured (24).

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Wild Heart -- Jade, 21:28:15 01/10/04 Sat
- I, �thoe, stop the sneak of Puck from the Wasteland. (NT) -- �thoe, 19:53:44 01/11/04 Sun