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Titre : Philosophical transactions of the Royal society of London. Series B, Biological sciences
Titre : Philosophical transactions of the Royal society of London. B
Auteur : Royal society (GB)
Éditeur : printed by Harrison and sons (London)
Éditeur : The Royal society (London)
Date d'édition : 1888-1990
Type : texte,publication en série imprimée
Langue : Anglais
Format : application/pdf
Identifiant : ark:/12148/cb34422226r/date
Identifiant : ISSN 00804622
Source : Bibliothèque nationale de France
Relation : http://catalogue.bnf.fr/ark:/12148/cb34422226r
Description : Variante(s) de titre : Philosophical transactions of the Royal society of London
Description : Variante(s) de titre : Philosophical transactions of the Royal society of London
Description : Périodicité : Irrégulier
Description : Etat de collection : 1888 (vol. 178)-1939
Provenance : bnf.fr
Date de mise en ligne : 13/01/2009
1908 (VOL199).
Page
(Screen 72 / 473)
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64 PROFESSORS J. T. WILSON AND J. P. HILL.
the anterior limit of the area pellucida, and even very slightly of t.he anterior limit
of the area of thick and eubical ectoderm.
The remainder of the wall of tKe blastodermic, vesicle. beyo^nd the région represented
'w the text-figure, is simply bilarainar, and does not differ in structure from that
found in the preceding gastrular stage. The surface-view of a portion of it,
as seen hy transmitted light under moderately high-power magnificat ion, is also
practically identical with that shown in the photomicrograph reproduced in fig. 8,
Plate 4, representing a portion of the bilaminar blastoderm from an egg belonging
to a somewhat later period of development.
In those mammalian embryos of which, at a corresponding stage, we possess the
most adequate descriptions, e.g., rabbit, pig, and dog, the differentiation of a definite
area of thick ectoderni to form the so-called embryonic shield seems to be a
constant phenomenon. In thèse forms thé original oval form of the shield gives
place to a more elongated one, soon- bëcommg shoe-sole-shaped. When this occurs
the posterior narrower part of the shield is found to possess a somewhat thinner
ectoderm than its more anterior portion (cf. Bonnet, '01). Partly, at least, in
virtue of this distinction, the posterior narrower part of the shield cornes to assume a
somewhat différent appearance to the rest of it and, as the development of the shoe-
sole-shaped shield proceeds, this differentiation extends forwards from the posterior
région alung its lateral margins (cf. Boxxet, loc. cit., Taf. xxxiv-v, figs. 20-23).
Bonnet identifies the marginal région thus arising as corresponding to thé parietal
zone," whilst the remainder of the shield, which is thicker, more opaque, and more
prominent, is stated by him to correspond to the "Stammzone" (paraxial zcne) on
either side of the axial structures. Parietalzone and Stammzone are, of
course, at bottom, mesodermal ditferentiations but they would appear also to
correspond to areas of early differentiation of "shield ectoderm in, e.g., the dog.
Now, in Ornithorhynchus, at the stage at présent under consideration, the
differentiation of cubical ectoderm is confined to an area which strictly coincides
with the Stammzone in the dog, as will be évident from a comparison of
Bonnets fig. 22 (loc. cit.) with our text-fig. 8. And, over the région which would
correspond to the "Parietalzone" in Boxnet's figs. 20-23, we hâve in Ornitho-
rhynchus only flattened squamous ectoderm, which passes over on each side, without
any distinction or demarcation, into the gênerai extra-embryonic ectoderm.
If, then, we proceed to define an embryonic shield in terms of thickened
cubical ectoderm in Ornithorhynchus, we find that the area so defined coincides–
not with the entire shield as differentiated in the other mammalian blastoderms
referred to– but only with the area of the Stammzone." The outer limit of our
area pellucida is seen in text-fig. 8 to lie not very far beyond the limits of this
ectodermally defined "Stammzone."
For the reasons just stated we are of opinion that the employment of the term
embryonic shield in the case of Ornithorhynchus is liable to lead to inaccurate
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