File:Analysis of development (1955) (17549778533).jpg

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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Ear and Nose 419 two posterior halves. Partial twinning, in- volving either the semicircular canals, which develop from the ventral half of the ear plate, or the saccule, which develops from the dorsal half, also occvurs" (Harrison, '45, pp. 295-296). On the basis of his experiments, Harrison has concluded that the ectoderm of the ear of harmonic equipotentiality after fixation of the AP axis and before that of the DV axis. These observations led to further in- vestigations on the polarity of the ear ecto- derm. In the experiments indicated above, both host and donor are at the same stage of development for any one experiment. As has cr.l
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p.a m.s TWO ANTERIOR HALVES TWO POSTERIOR HALVES Fig. 149. Normal right labyrinth and two types of enantiomorphic twins, medial surface. The large arrow points to the posterior end of the embryo; the small arrows point posteriorly with respect to the labyrinth or half labyrinth to which they refer, c.a. Anterior semicircular canal; c.p, posterior canal; cr.a and cr.p, the corresponding sensory cristae; cr.l, lateral crista, projected from the ventrolateral wall but not visible in this view; d.e, endolymphatic duct; m.l, macula of lagena; m..s, macula of sacculus; m.u, macula of utriculus; p.a, papilla amphibiorimi; s.e, endolymphatic sac (Harrison, '45). region and the surrounding area is at first isotropic about the axis perpendicular to its surface. Just as the neural folds are closing a change supervenes. The ear plate becomes polarized with respect to its AP axis. A little later conditions again change, indicating a transition from an indifferent DV axis to one which is definitely polarized. A correla- tion between the polarization of the ear ectoderm and a regularity in the molecular arrangement of the constituent cells has been postulated and an attempt to demon- strate this by x-ray diffraction, though un- successful, indicates a possible means of extending fundamental knowledge of em- bryology (Harrison, Astbury, and Rvxdall, '40). In Amblystoma Hall ('37) studied laby- rinths whose ectoderm had been rotated 90 degrees. In these experiments, the ear rudi- ment may still respond to some of the tests been indicated, conditions in the hosts as well as the grafts are variables. When the stage of either host or donor is constant in a series, a further analysis of the problem is possible; this has been done in hetero- stadic transplants to homeotopic and hetero- topic positions in Amblystoma (Hall, '39, '41; Yntema, '39). The results of these ex- periments call for modifications in the con- clusions drawn from homostadic operations (Fig. 148B-£). Expression of polarity de- pends in part on the strength of polarization in the rudiment, in part on the intensity of the polarizing factors in the environment. A labile AP polarity is found in the ear rudi- ment of the early neural plate stage; this anisotropism is masked in Harrison's experi- ments by the ability of the hosts used to reverse it. A similar inclination along the DV axis was not demonstrated. Host factors concerned with the arrangement of struc-

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