Paper - Description of a Human Embryo of Twenty-three Paired Somites
Peter Thompson
<pubmed>17232726</pubmed>| PMC1289111
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Introduction
THE embryo which forms the basis of this work came from Dr Robert Meyer's collection in Berlin. Sent to Professor Keibel, who was accumulating material for his forthcoming Normentafeln of human embryos, the specimen was most kindly lent to me with the object of making a model whilst I was at the Anatomical Institute in Freiburg during the summer of 1906. Working with this specimen, I had an excellent opportunity of becoming acquainted with the reconstruction of embryos by the wax-plate method, as carried out so successfully in that University.
The embryo, obtained at an operation, was recorded as 2.5 mm. long, and was cut transversely into 488 sections, each 5 , in thickness, and stained with borax-carmine. In making the model, every other section was drawn, enlarged 100 diameters, and the wax plates were made 1 millimetre in thick- ness. Whentheplateshadbeencutandlaidinposition,itwasfound, owingr prol)ably to the hot weather and the weight of wax, that the total height of the model was only 220 millimetres, and, in order to correct the error, 24 additional wax plates were made, duplicates of every tenth section, and introduced into the series. In this way the total length of the model was brought up to 244 millimetres, corresponding to the 244 sections used, which indicates a shortage of less than 3 per cent, when compared with the 250 millimetres, the absolutely accurate measurement which the enlargement should have been, taking the length of embryo as 25 min. The difference is probably due to a slight shrinkage, which would most probably occur in preparing the specimen for cutting. It may be stated here that the embryo is histologically in excellent condition, mitosis being readily observed in the multiplying cells, and there seems no valid reason for doubting that the specimen is a normal one. Inaddition to the model of the whole embryo and its yolk sac, other models were made of special organs, namely, the heart and its endothelial tube, the brain, a part of the alimentary canal, and the septum transversum.
It may be noted that the embryo described in this paper resembles in many ways His's embryo Lg., which was2-15mm. in length and estimated to be about fifteen days old.
Descriptions of Models
The Embbryo and its Yolk Sac
The head is small, somewhat flattened from above downwards, and pointed. At the side, the opening of the otocyst externally is seen over the upper end of the second post-oral bronchial cleft. Anteriorly, two slight but well-marked bulgings indicate the position of the optic vesicles, between which is a depressed area corresponding to the anterior neuropore, com- pletely closed, but stil recognizable in the sections by the continuity of the general ectoderm and the neural ectoderm. The mouth is a narrow transverse cleft, at the bottom of which the bucco-pharyngeal membrane is seen, perforated in two or three places. Behind the mouth three bronchial clefts are seen externally, and the large prominence below it indicates the position of the heart. There is no trace of limbs.
The alimentary canal is in wide communication with the yolk sac through the vitelline duct, which, with the body stalk, occupies the umbilical orifice. The yolk sac, about as large as the embryo, is globular in form and presents two marked depressions, an upper one for the accommodation of the head, and the prominence of the heart, and a lower one for thetailandthebodystalk. Thebodystalk,aboutthesamethicknessas the vitelline duct, below and to the left side of which it is placed as it emerges from the umbilical orifice, is directed forwards, downwards, and outwardstojointhechorion. Inthecaudalregionthebackoftheembryo curves to end in the prominent tail which is sharply flexed and turned to the right, so that the embryo appears to be somewhat spirally twisted. Here the neural groove is not yet closed, giving rise to a posterior neuro- pore, a gutter, shallow at first but gradually deepening up to the point where it becomes continuous with the central canal of the spinal cord just infrontofthebendoftherump. Thecloacalmembraneisclearlydistin- guishable on the side opposite to the neural groove, and the free extremity ofthecaudaisreceivedintoadepressionontherightsideofthebodystalk, intoalitlecul-de-sacformedbytheamnion. Atthecurveoftherump the mesoblastic somites are recognizable as distinct protuberances, whereas in the region of the back and the neck they are much less prominent. Root of Amnion.-Caudalwards, the root of the amnion is prolonged on to the dorsal aspect of the body stalk, where it has a V-shaped attachment. Thetwolimbsarecontinuedupwardsoneithersideofthe umbilical orifice, that on the right passing between the tail and the stalk and forming, as mentioned above, the recess into which the tip of the tail isreceived. Attheupper(anterior)boundaryoftheumbilicalorificethe amnion is reflected along a line which runs transversely across the embryo at the lower end of the heart prominence, and in this way the attachment oftherootoftheamnioniscompleted. Itwillthusbeseenthat,inthis embryo, a very small part only of the body stalk, on its dorsal surface, is covered with ectoderm.
Nervous System
The nervous system is closed except in the region of the tail, where, as already noted, there is a posterior neuropore. Whilst, therefore, the caudal portion of the neural tube is the last part to close in this embryo,this is not invariably the rule. Indeed there seems to be considerable variations both with regard to the last place of closure of the tube, and the time at which the closure is complete.
According to Hertwig's Hanzdbuch, the medullary groove in the human embryo at the end of the second week is not yet closed, and attention is drawn to the fact that the series, from the second week to the time when the closure is complete, have not yet been described. The specimen under consideration forms another link in that series, of which a few may be briefly noted.
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Cite this page: Hill, M.A. (2024, June 2) Embryology Paper - Description of a Human Embryo of Twenty-three Paired Somites. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Description_of_a_Human_Embryo_of_Twenty-three_Paired_Somites
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