. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. DEVELOPMENT OF THE DIGESTIVE TRACT 281 O-A contact with the ectoderm to form the cloacal membrane (Fig. 170, cm) lies a little in front of the actual end of the archenteric cylinder, the portion of the latter which lies posterior to the membrane forming what has been termed the postanal gut {). This diminishes in size during development and early disappears altogether, and the pouch-like fold seen in Fig. 170 between the intestinal portion of the archenteron and the allantoic stalk (al) d
. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. DEVELOPMENT OF THE DIGESTIVE TRACT 281 O-A contact with the ectoderm to form the cloacal membrane (Fig. 170, cm) lies a little in front of the actual end of the archenteric cylinder, the portion of the latter which lies posterior to the membrane forming what has been termed the postanal gut {). This diminishes in size during development and early disappears altogether, and the pouch-like fold seen in Fig. 170 between the intestinal portion of the archenteron and the allantoic stalk (al) deepening until its floor comes into contact with the cloa- cal membrane, the cloaca be- comes divided into a ventral por- tion, with which the allantois and the primitive excretory ducts (w) are connected, and a dorsal portion which becomes the lower end of the rectum. This latter abuts upon the dorsal portion of the cloacal membrane, and this eventually ruptures, so that the posterior communication of the archenteron with the exterior becomes established. This rup- ture, however, does not occur un- til a comparatively late period of development, until after the em- bryo has reached the fetal stage; nor does the position of the membrane correspond with the adult anus, since later there is a considerable development of mesoderm around the mouth of the cloaca, bulging out, as it were, the sur- rounding ectoderm, more especially anteriorly where it forms the large genital tubercle (see Chapter XIII), and posteriorly where it pro- duces the anal tubercle. This appears as a rounded elevation on each side of the median line, immediately behind the cloacal mem- brane and separated from the root of the caudal projection by a de- pression, the precaudal recess. Later the two elevations unite across the median line to form a transverse ridge, the ends of which curve. Fig. 169.—Reconstruction of the Anterior Portion of an Embryo of MM. ab. Aortic bulb; h, heart; 0, auditory cap- su
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Keywords: ., bookautho, bookcentury1900, bookdecade1910, booksubjectembryology
