44 HUMAN ANATOMY. 



as already noted, includes the allantois, with its blood-vessels, and diverticulum, 

 while traces of the vitelline circulation are for a time visible within the atrophied 

 walls of the umbilical duct. As gestation advances, the amnion and the chorion 

 become closely related, but not inseparably united ; between these attenuated mem- 

 branes lie the remains of the once voluminous yolk-sac, which at birth appears as 

 an inconspicuous vesicle, from three to ten millimetres in diameter, situated usually 

 several centimetres beyond the insertion of the umbilical cord. 



In cases in which the closure and the obliteration of the vitelline duct before birth 

 are imperfectly effected, a portion, or even the whole, of the intra-embryonic segment 

 of the canal may persist as a pervious tube. Although in extreme cases of faulty 

 closure a passage may lead from the digestive tube to the umbilicus, and later open 

 upon the exterior of the body as a congenital umbilical anus, the retention of the 

 lumen of the vitelline duct is usually much less extensive, being limited to the prox- 

 imal end of the canal, where it is known as Meckel' s diverticulum. The latter is con- 

 nected with the ileum at a point most frequently about 82 centimetres (thirty-two 

 inches) from the ileo-caecal valve. Such diverticula usually measure from five to 

 7.5 centimetres in length, and possess a lumen similar to that of the intestine with 

 which they communicate. 



The foregoing envelopes, the amnion and the chorion, are the product of the 

 embryo itself ; their especial purpose, in addition to affording protection for the deli- 

 cate organism, is to aid in establishing close nutritive relations between the embryo 

 and the maternal tissues, which, coincidently with the development of the foetal 

 envelopes, undergo profound modifications ; these changes must next be considered. 



The Deciduse. — The birth of the child is followed by the expulsion of the 

 after-birth, consisting of the membranes and the placenta, which are separated from 

 the uterine wall by the contractions of this powerful muscular organ. Close inspection 

 of the inner surface of the uterus and of the opposed outer surface of the extruded 

 after-birth shows that these surfaces are not smooth, but roughened, presenting evi- 

 dences of forcible separation. The fact that the external layer of the expelled after- 

 birth consists of the greater portion of the modified mucous membrane which is 

 stripped of? at the close of parturition suggested the name decidiice for the mater- 

 nal portion of the foetal envelopes shed at birth. 



Since the deciduae are directly derived from the uterine mucous membrane, a 

 brief sketch of the normal character of the last-named structure appropriately pre- 

 cedes a description of the changes induced by pregnancy. The normal mucous 

 membrane lining the body of the human uterus (Fig. 51) presents a smooth, soft, 

 velvety surface, of a dull reddish color, and measures about one millimetre in thick- 

 ness. The free inner surface is covered with columnar epithelium (said to be cili- 

 ated) which is continued directly into the uterine glands. The latter, somewhat 

 sparingly distributed, are cylindrical, slightly spiral depressions, the simple or bifur- 

 cated blind extremities of which extend into the deeper parts of the mucosa in close 

 relation to the inner bundles of involuntary muscle ; all parts of the tubular uterine 

 glands are lined by the columnar epithelium. The muscular bundles representing 

 the muscularis mucosae are enormously hypertrophied and constitute the greater 

 part of the inner more or less regularly disposed circular layer of the uterine muscle. 

 The unusual development of the muscular tissue of the mucous membrane reduces 

 the submucous tissue to such an insignificant structure that the submucosa is gener- 

 ally regarded as wanting, the extremities of the uterine glands being described as 

 reaching the muscular tunic. The glands lie embedded in the connective-tissue 

 complex, rich in connective-tissue elements and lymphatic spaces, that forms the 

 tunica propria of the mucosa. 



With the beginning of pregnancy the uterine mucous membrane undergoes 

 marked hypertrophy, becoming much thicker, more vascular, and beset with nu- 

 merous irregularities of its free surface caused by the elevations of the soft spongy 

 component tissue. These changes take place during the descent of the fertilized 

 ovum along the oviduct and indicate the active preparation of the uterus for the 

 reception of the ovum. 



According to the classical description of the encapsulation of the ovum (Fig. 52) 

 by the uterine mucous membrane, the embryonic vesicle becomes arrested within 



