Difference between revisions of "Paper - A human embryo of seven to eight somites"

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#REDIRECT [[Paper - A human embryo of seven to eight somites (1917)]]
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| [[File:Mark_Hill.jpg|50px]]  This 1917 paper presented by Evans and Bartelmez at the 33rd Meeting of the American Association of Anatomists in New York (Dec 27-29, 2016). Describes two embryos that had been serially sectioned.
 
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=A Human Embryo of Seven to Eight Somites=
 
Evans HM. and Bartelmez GW.
 
 
 
Department of Embryology, Carnegie Institution of Washington and Department of Anatomy, University of Chicago.
 
 
 
 
 
This embryo was obtained from an aborted ovum measuring 18.0 by 13.0 by 10.8 mm. including the villi, fixed intact in 10 per cent formalin. The age was estimated clinically as three weeks. The embryo measured 2 mm. in length in formol and belongs in the group with the Mall embryo no. 391 (described by Dandy), the seven somite Spee embryo, the 2.11 mm. embryo of Etternodand the R. Meyer embryo no. 335.
 
 
 
 
 
The embryo was found attached directly opposite the chorion laeve, projecting into the extraembryonic coelom at right angles to the chorionic wall, supported by a few strands of magma. The embryo lies flat upon the yolk sac, with head and tail folds rising above it. The height of the head fold may have been slightly increased by distortion in the fixative but this region could not possibly have been bent ventrally over the yolk sac as it is in the Keibel embryo of 6 somites (Normentafel no. 3), and the above mentioned Eternod specimen. The amniotic cavity is large and the amniotic duct extends along the belly stalk. In dorsal view the embryo appears somewhat like a spoon, the expanded cephalic neural folds corresponding to the bowl. The neural tube is closed from the middle of the hind brain to the level of the seventh somite. Caudally the neural folds gradually flatten out and pass over into the primitive streak at Henson’s node. The neurenteric canal has already closed. It is possible to delimit forebrain, midbrain and three hind brain neuromeres since the cerebral flexure has just begun to appear, the neural crest is actively proliferating and the otic plate and ganglion are well defined. The asymmetric forebrain is bent almost at right angles to the hindbrain, the midbrain forming the knee. In the forebrain two shallow sulci can be distinguished converging rostrally: they are the earliest stage of the optic sulci yet described in man. The neural crest cells are migrating from the dorsolateral region of the folds in the midbrain and hind brain as far caudally as the VII-VIII ganglion. This is a bulbar swelling of the neural fold dorsally, lying opposite the thickened otic plate.
 
 
 
 
 
The pharynx is intermediate in development between that of 391 in the Mall collection and 335 of R. Meyer. Its epithelium is in contact with the ectoderm of the oral membrane, the first visceral pouch is well developed and its dorsal diverticulum touches the thickened ectoderm at one point. The second pouch is beginning to form and there is an asymmetric thyroid anlage. The hind gut extends but five sections caudal to the origin of the allantois and there is no cloacal membrane. The chorda begins near the upper end of the pharynx as a thickened ridge and is everywhere incorporated in the entoderm except in the region of Henson's node.
 
 
 
 
 
The heart is an almost bilaterally symmetrical tube formed by the union of the vitello-umbilical veins lying ventral to the pharynx. A pair of delicate vessels, the first aortic arches pass around in front of the first pair of visceral pouches from the bulbar end of the heart to the greatly dilated cephalic ends of the dorsal aortae. The latter have four well developed pairs of dorsal rami, the first two of which are growing in the direction of the fifth and eighth ganglia respectively. Caudally the aortae break up into a plexus on the dorsal wall of the yolk sac which plexus in turn gives rise to the umbilical arteries at the beginning of the belly stalk. More rostrally the vitelline vein is beginning to differentiate from the plexus on the yolk sac.
 
 
 
 
 
A comparison of the various embryos of this stage shows that the different systems of organs do not develop pari passu and it is impossible to arrange thein correctly in a series by referring to a single character such as number of somites or greatest length. In the present case the nervous system is relatively more differentiated than any other. It is necessary to seriate a limited group of embryos separately for each organ system.
 
 
 
 
 
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[[Category:1910's]][[Category:Carnegie Stage 10]][[Category:Week 4]][[Category:Historic Embryology]]
 

Latest revision as of 10:49, 30 July 2020