At this stage of its evolution, the intestinal canal may be conveniently described as con sisting of three portions : an anterior, which extends from the beginning of the tube to the umbilical coil ; a middle, which is formed by this coil itself; and a posterior, which reaches from the latter segment to the end of the canal.
The anterior of these three portions may again be subdivided into three similar seg ments. The first, which gradually elongates from the blind end that was formerly the fovea cardiaca, is developed during the evolu tion of the thorax, so as to form the ceso phagus. And it finally opens into the cavity of the mouth ; which is itself developed from an involution of the skin, and from the united ends of the anterior visceral arch. The second or middle segment dilates, turns on its left side, and then bends transversely to the axis of the body, to form the stomach. The pyloric valve is only visible some time after this change has occurred. And the third or lowest portion of this anterior segment is con verted into the duodenum.
The middle, umbilical, or extra-abdominal, part of the canal, is developed into the jeju num and ileum, the ccecurn, the vermiform appendix, and part of the colon. In this process, the change of form undergone by the small intestine is limited to a mere increase in its length and in the degree of its con volution : — an alteration which is accom panied by a further elongation of its mesen tery. The upper boundary of the large intestine is first seen as a constriction and change of calibre, which occupy a point some distance below the insertion of the umbilical duct. Such a situation of the future ccecum conclusively shows, that the vermiform appendix is not that permanent intestinal end of the duct, which Oken supposed it to be. This commencement of the large intestine next enlarges into a projecting pouch of uniform width. But the lower end of this pouch soon ceases to enlarge, and remains as the vermiform appen dix. While its upper part, increasing in size, becomes the ccecum. The valve appears at about the tenth week. But the proper shape and size of the ccecum are only acquired towards the end of fcetal life.
The colon is developed from the lower part of the second, and the upper part of the third portion of the rudimentary intes tine. The ascending colon is at first a simple btraight tube, which, commencing in the pouch just alluded to, runs forwards along the spinal column, lying to the left of the numerous coils of the small intestine. The succeeding backward bend of this tube has at first a median position, which renders it par allel with (and close to) the ascending colon.
But this part of the canal soon elongates ; and, passing outwards towards the left side, forms the transverse and descending portions, as well as the sigmoid flexure, of the colon. Finally, the blind end which corresponds to the rec tum is continually moved downwards by a gradual lengthening of the tube; so that it meets, and at last opens into, a cavity, which is sent inwards from the skin to form the future anus. The sacculation of the large intestine only occurs in the latter half of uterine life. The valvulw conniventes appear still later, and are but rudimentary at birth.
The development of the various microscopic constituents of the canal may be almost as briefly summed up. The cell-growth (which is derived from the mucous lamina), and the fibrous tunic (which is developed froin the vascular lamina), are at first very loosely united to each other. Hence they- may be easily separated into distinct and comparatively plane strata ; of which the fibrous has about double or treble the thickness of the epithelial one. The cells of the latter affect an elongated or columnar form at a very early date of fcetal life (about the sixth week). The various offshoots of tubes and other glands which are contained in the wall of the canal, are developed from a mass of cell growth, which sprouts from the external surface of the MUCOUS layer, and gradually acquires the definite form and cavitary ar rangement specific to these minute structures. The larger accessory glands of the liver and pancreas are produced from a similar mass which lies external to the bowel: and they ultimately prolong their ducts so as to open into the cavity of the intestine.
The fibrous layer, which is at first smooth and homogeneous, soon becomes roughened into little projections, which ultimately take the shape of conical processes. These, as they enlarge, pass upwards into the mucous or epithelial layer. Some of these projections not only separate the various tubes and glands from each other, but, by a farther advance and enlargement, carry before them the general surface of the cell-growth. They thus form the future villi. While others — and by far the majority — affect a lateral, instead of a vertical, growth ; uniting with their neighbours by cross ridges, which soon form a network, that extends between the tubes at all parts of their height, so as to constitute a matrix for these and the other structures derived from the mucous lamina.
Finally, the unstriped muscular fibres, and the white and yellow fibrous elements, repeat the ordinary steps seen in the development of these tissues generally.