Resumen por el autor, Howard B. Adelmann.La significacih de la placa precordal: L?n estudio interpretativo. E n adici6n a1 mesoderrno gastral y peristomal de Rabl debe reconocerse la existencia de un mesoderm0 preaxial continuo desde ambos lados por delante del notocordio. El autor denomina placa precordal a este mesoderrno preaxial. La placa precordal es, por consiguiente, una estructura esencialmente mesodhmica.El mesoderrno preaxial representa inesodermo formado en el labio primitivamente dorsal o labio cranial del blastoporo. La relaci6n del mesoderrno preaxial con la teoria general del mesodermo es objeto de discusi6n en el presente trabajo. E n el tibur6n y en otras formas en las cuales se forman somitas premandibulares la continuidad preaxial del mesoderrno se expresa de un modo sorprendente mediante la conexih media (" Querbriicke ") de 10s somitns preniandibulares. La placa precordal del tibur6n produce 10s somitas premandibulares, que se originan a expensas de una sola masa preaxial de mesodermo, y que e s t h separados entre si por un crecimiento bilateral semejante a1 que tiene lugar en toda la cabeza. La continuidad del mesoderrno situado delante del notocordio es comparable a la continuidad del material nervioso situado delante de la sutura neurocordal de His, descrita por Kingsbury ('20).La placa precordal no produce material notocordal en el pollo o en el tibur6n per0 es un manantial importante del mesenquima. La relaci6n de la placa precordal con el gradiente metab6lico es objeto de discusi6n. Un in6nstruo epignato puede explicarse como el resultado de la persistencia de nlgunn parte del material indiferente de la plnca precordal.Tirtnzlation by Jo5E F. Vonidcz Coriicll Mcilicul Collqg~, S e a Yo.1.
Faun TEXT FIGURES AND EIGHT PLATES AU'FHOPU'S ARSTRACTThe anlage of the abducens musculature appears first a t 24-25 somites a s a condensation situated dorsal to the mandibular arch.The anlage of the superior oblique grows forward from a mesodermal condensation situated in the maxillomandibular region, termed for convenience the maxillomandibular condensation. This last consists of three parts:1) the anlage of the superior oblique: 2 ) the anlage of the abducens musculature, and, 3) a n intermediate region.The intermediate portion of the maxillomandibular mass forms a condensation with which the anlage of the abducens musculature fuses. Its fate is, therefore, similar to that of the so-called 'muscle E' of elasmobranchs, which has been described as fusing with the lateral rectus.How much muscle is formed from the intermediate condensation in the chick has not been determined.The development of the pyramidalis a n d quadratus nictitans muscles, derivatives of t,he abducens complex, is described.The premandibular head cavities are replaced hy solid mesodermal condensations, on the surface of which the anlagen of the oculornotor muscles appear. The premandihular mass expands laterally a n d anteriorly over the hulbus, carrying the oculomotor muscles to their respective positions on the bulbus.Portions of the premandibular and maxillomandibular condensations not inrolved in eye-muscle formation take part in the formation of choroid and sclera.The growth shiftings of the eye m i i d e s are analyzed. The order of their appearance is commented upon. INTROUUCTIOh"It is, perhaps, safe to say that no other group of muscles has aroused more interest among morphologists and embryologists than the muscles of the vertebrate eye. It is, of course, well known that they are typically six in number. Four of them, the superior rectus, the inferior rectus, the medial rectus, and the inferior oblique, are innervated by the oculomotor nerve. The superior oblique is innervated by the trochlear and the lateral rectus by the abducens. In reptiles and certain mammals the abduceiis innervates an additional muscle, the retractor bulbi, which is represented in birds by two muscles, the pyramidalis and the quadratus nictitans.
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