Intense Vibration

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225 the yolk-sac is cut tangentially, but it is evident that toward the allantois its cells are cuboidal. The allantois consists of similar cells and contains a lumen. Over the greater part of the yolk-sac, however, the entoderm forms a very thin layer resembling endothelium, precisely as recorded by Beneke. In the most ventral portion there are occasional cuboidal cells with large round nuclei and protoplasm which projects above the general level into the cavity of the yolk-sac.

– The pharyngeal membrane as figured by His. X37 diam. A, embryo “Lg,” 2.15 mm.; B, embryo “BB,” 3.2 mm.

Ill, IV, epithelial bodies of the third and fourth pharyngeal pouches; Hy., hyoid arch (operculum); Lar., anlage of the larynx; Per., pericardial cavity; S. c, sinus cervicalis; Sy., sympathicus; Thym. Ill, thymus anlage of the third pharyngeal pouch; Thyr., thyreoid; Tr., trachea; ub.

342. X 100.

They usually become detached and degenerate. It is possible that accessory pancreases sometimes develop in relation with the embryonic intestinal diverticula described in a previous section. The encircling of the portal vein by a process of the dorsal pancreas has apparently not been observed in man, though characteristic of the rabbit and pig (Thyng). The common bile-duct occupies a groove in the head of the pancreas which is frequently converted into a canal of pancreatic tissue (see Helly, 1898).

The denser grouping of the embryonic thyreoid cells in the lateral lobes of somewhat later stages (see p. 469), which these authors have also noticed, is not to be referred to the ultimobranchial bodies, as they have supposed. The (unpaired, middle) thyreoid anlage has been known since Rathke’s time. That a derivative of the pharyngeal pouch region becomes associated with the thyreoid anlage in mammals was first observed by Wolfler (1880), and firmly established by Stieda (1881) and Born (1883) ; from the latter comes also the term lateral or posterior thyreoid anlage, which has been applied to the untimobranchial body, but which is rejected in the account given above. must receive some further consideration (see above, p. 453). A thorough exposition of the question has been given by Tandler (1909), who, however, had for study no material showing the pouch with an epithelial closing membrane (compare in this respect Harmnar in the Normentafel).

Thus, that of the third pouch may remain even in man near its place of origin, not far from the division of the carotid (p. 460), or, on the other hand, it may descend into the thoracic cavity with the thymus. A diminution in the number of epithelial bodies is very difficult to demonstrate, on account of the possible occurrence of anomalies in position ; increase, probably by division of the anlagen, was first observed by Kursteiner and has since been repeatedly seen; Zuckerkandl has described a case in which there were eight, and Erdheim one with eight and one with twelve.

In addition there were also small cysts which were not formed of thyreoid tissue. According to the same authoress, struma? may arise from the parathyreoids or from the ultimobranchial bodies as well as from the thyreoid, whence the form variability of these tumors. According to H. Rabl (1000).

.4, in an embryo of 13.6 mm. (Harvard Collection, Series 839). X 55 diam. (From a model by F. W. Thyng.) B, in an adult. (After C. M. Jackson.) In B the outline of the pancreas is dotted.

He described them as having round and lightly staining nuclei, with relatively abundant protoplasm which stains deeply with eosin. At this stage the islands are not penetrated by blood-vessels.

They are partly surrounded by capillaries. The difficulty of distinguishing these developing islands from the alveolar buds is apparent in sections and in the published figures. Laguesse admits this difficulty, but he concludes that certain formations in sheep embryos, apparently comparable with those seen in Fig. 312, are the first stages in the development of the islands. An earlier stage of the islands than that shown in Fig.

Lewis (1903) found similar detached cysts in the liver of a 12 mm. pig, and considered them to be cut off from the secondary hepatic ducts. Whether they are detached portions of the hepatic ducts is questionable. They may arise in situ by a transformation of the cells of the hepatic trabecular The Periportal Ducts.- In an embryo of 22.8 mm. (Fig. 293, A) the spread of the bile-ducts along the main branches of the portal vein has begun.

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The tunica propria becomes gradually differentiated before the muscularis mucosae has appeared. It is a dense layer of mesenchyma at 99 mm. The lymphatic vessels, which in earlier stages were present in the mesentery, are now found in the submucosa, but they cannot be seen in the propria. At 240 mm. both solitary and aggregate nodules of lymphoid tissue have appeared in the tunica propria. Their relation to the lymphatic vessels could not be determined in the specimens studied.

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Forssner (1907) agrees with Meckel and Berry. Kolliker (1861), on the contrary, states that the villi arise in the beginning of the third month as wart-like outgrowths of the mesenchymal layer, which push the epithelium before them and become cylindrical. This was confirmed by Barth (1868). Brand (1877) found scattered villi at one and a half months. Voigt (1899), by means of reconstructions of pig embryos, found that depressions and furrows develop on the free surface of the epithelium, marking out areas of greater diameter than the future villi.

Talk:Book – Manual of Human Embryology 17

In the 2.11 mm. specimen described by Eternod (Fig. 234, A ) there is a short prolongation of the chordal canal beyond the neurenteric canal, but there is apparently no other evidence that the notochord ever extends beyond the primitive knot. In a 2.1 mm. specimen described by Mall the obliterated neurenteric 6 It may be noted that His described the medullary groove as extending along the body-stalk. Anatomie menschlicher Embryonen, iii, p. 224.

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