Title: Analysis of development
Identifier: analysisofdevelo00will (find matches)
Year: 1955 (1950s)
Authors: Willier, Benjamin H. (Benjamin Harrison), b. 1890
Subjects: Embryology; Embryology
Publisher: Philadelphia, Saunders
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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Limb and Girdle 435 of the posture assumed by the free extremity. In the diagram (Fig. 154) the nearly central region of the limb area (between 3 and 5) is comparatively free of girdle-form- ing cells. Gradually the limb posture be- comes dominated by the girdle structiires at its proximal extremity. This is shown by experiments in which the developing tissues around the limb area are rotated, producing a corresponding rotation of the limb without reversing its laterality. In this process of postural control the outlying rudiments must presumptively be considered as the most effective factors. When the limb is trans- tated the girdle may, instead of showing its normal morphological components, de- velop as a plate-like sheet of cartilage. Since the forelimb mesoderm will grow into a forelimb under various experimental conditions, including the inversion of the mediolateral axis, why is it that a limb disc does not always develop two limbs, as experimentally demonstrated by Nieuw- koop ('46) in embryos deprived of the yolk mass? It is, of course, easy to assume a simple mechanical inhibition caused by the presence of the heavy yolk. When, how- ever, a small bit of head ectoderm is placed
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Fig. 154. The various portions of the girdle-forming region have been tested by transplantation experi- ments for which the parts marked out by the vertical lines (a to e) and the horizontal lines (i to 7) were employed singly or in combination (from Detwiler, '18; Swett, '23; and Nicholas, unpublished). planted heterotopically with but a small part of the scapula, the imparted rotation is not corrected in later stages. The relation between the girdle and the free extremity is important also in redupli- cation, reversal of symmetry and inversion. The work of Swett ('32, '45) throws some light on these problems but much remains to be explained. Usually the girdle is ab- normal when the limb is duplicated after operations involving rotations of various de- grees. Usually after orthotopic inverted limb disc transplantations the limb is reversed in its asymmetry and so is its girdle, i.e., a right limb disc upside down on the right side of the embryo becomes a left limb with a left-sided girdle. This speaks strongly for a change in the mosaic structvire of the gir- dle parts, for their localized developmental potencies cannot be irreversibly fixed or determined. When areas as large as 5 somites in diameter are used for transplan- tation there is seldom formed a reduplicat- ing limb, whether orthotopically or hetero- topically transplanted. If this graft is ro- over the limb mesoderm, replacing its nor- mal covering ectoderm, a similar result is obtained, and here the mechanical effect can- not be assumed to be perceptibly greater than in the normal ectoderm. Yet there is inhibition of differentiation. The presence of the developing girdle might also be thought of as a possible factor that causes the ex- tremity to develop outside the flank instead of inside the coelom, but it lacks the ability to make the mesoderm form a free extremity when covered with head ectoderm. There are still many problems of morphogenesis to which the limb-girdle combination may con- tribute solutions. THE PELVIC GIRDLE Stultz ('36) has performed a series of ex- periments which yield information regarding the development of the hindlimb and girdle of Amblystoma. The same course of events is followed as in the forelimb but the mo- saic constitution of the girdle is not so striking as in the forelimb. In his analysis

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