File:Analysis of development (1955) (17549650873).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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Teleosts and Birds 309 culiarities of the ectoderm at the tips of the limb buds in the chick, point to how much there remains to learn about the prop- erties of non-neural ectoderm. the chorda field in the chick gastrula (Fig. 117E,F) is in striking contrast, and would indicate that this structure, at least, is local- ized relatively early in the latter form—a THE LOWER LAYERS As gastrulation progresses, not only do in- duction capacity, neural competence, and the specific competences for different levels in the ectoderm become gradually restricted to the appropriate embryonic area: the invaginating layers themselves ultimately must become a mosaic of areas of definite differentiation tendencies. It is believed that all these areas have at first the properties of fields, of indefinite boundaries, possibly overlapping one another. Toward the time of beginning morphogenesis, the boundaries of the fields evidently become more restricted and more precise, as if brought into focus by some microscope capable of reading the future; each field then coincides with the corresponding organ-forming area. In the forms vmder discussion, our experi- mental knowledge of this course of events in the lower layers is based only on the results of transplantation experiments in which at least two layers were involved, and which resulted in differentiation of complexes of several tissues. The desirable isolation ex- periment has not yet been technically pos- sible. The succession of figures which form the matter of the present section is an ab- straction, presented in order to help the reader to visualize, for each major embryonic structure, the spatial distribution of its origin in grafts, delimited as well as the experimental material allows. Many of the studies on which these abstractions are based have not been carried through to the stage where the potency field corresponds with the actual organ-forming area; but the tendency in that direction is clearly observ- able. Figure 117 deals with the axial mesoderm, in so far as notochord and somites (or skele- tal muscle) are found to occur differentially in grafts of parts of blastoderms. The re- ported data are unsatisfactory for the blas- tula stage; in particular the distributions indicated for the chick (Fig. 117D,J) in the unincubated stage are based on a few positive cases and may not be valid. The trout, however, shows an vmderstandable sequence, with a notochord field converging toward the dorsal lip (Fig. 117B,C), fol- lowed by later invaginated somite material (Fig. 117//,/). The strict delimitation of
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Fig. 117. Progressive changes in fields for pro- duction of chorda and somites in grafts from Salmo (Luther, '36a) and the chick (Butler, '35; Hunt, '31, '32; Rudnick, '32; Rawles, '36). In the diagrams for SaLmo, the left half of the figure shows the actual percentages of differentiation in various sec- tors, on which the scheme of the right half is based. No such quantitative estimate has been attempted in the case of the chick. Localization in blastula stages is based on very incomplete information, which may be of no significance. The marked re- striction of chorda-forming material in the chick (E,F) is based on reliable data. The extent of the somite field in the chick (J,K,L) is based merely on differentiation of skeletal muscle, and so is not critical, nor are the lateral boundaries established. condition not reached even in the late gas- trula in Salmo. The interpretation of this localization in the chick is not easy, as examination of Fig- ure 118 will lead us to reflect. Here we have similar figures showing progressive lo- calization of two more lateral mesodermal

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