920 GENERATION. 



tremity, which is closed over in front for a short distance. The closure of the abdominal 

 plates then extends laterally and from the two extremities of the intestine, until we have 

 only the opening remaining for the passage of the umbilical cord and the pedicle of the 

 umbilical vesicle. There is at first an open communication between the lower part of the 

 intestinal tube and the allantois, which forms the canal known as the urachus ; but that 

 portion of this communication which remains enclosed in the abdominal cavity becomes 

 separated from the urachus, is dilated, and eventually forms the urinary bladder. When 

 the bladder is first shut off, it communicates with the lower portion of the intestine, which 

 is called the cloaca ; but it finally loses this connection and presents a special opening? 

 the urethra. 



As development advances, the intestine grows rapidly in length and becomes convo- 

 luted. It is held loosely to the spinal column by the mesentery, a fold of the peritoneum, 



this membrane being reflected along the walls of the ab- 

 dominal cavity. In the early stages of development, a por- 

 tion of the intestine protrudes at the umbilicus, where the 

 first intestinal convolution appears ; and sometimes there 

 is a congenital hernia of this kind at birth, which usually 

 disappears under the influence of gentle and continued 

 pressure. An illustration of this is given in Fig. 301. This 

 protrusion, in the normal process of development, is grad- 

 ually returned into the abdomen, as the cavity of the pedi- 

 cle of the umbilical vesicle is obliterated, at about the tenth 

 week. 



At the upper part of the abdominal cavity, the aliment- 

 i pig, showing a loop arv canal presents two lateral projections, or pouches. 

 of intestine, farming an umUii- The one on the left side, as it increases in size, becomes the 



eat herma. From a specimen in 



the possession of Prof. Daiton. greater pouch of the stomach, and the one on the right 



From the convexity of the loop a thin s jd e the lesser pouch, 

 filament is seen passing to the 



umbilical vesicle, "which is here At a short distance below the attachment of the pedicle 

 of the umbilical vesicle to the intestine, there appears a 



rounded diverticulum, which is eventually developed into the caecum, or the commence- 

 ment of the larger intestine. The caecum gradually recedes from the neighborhood of the 

 umbilicus, which is its original situation, and finally becomes fixed, by a shortening of the 

 mesentery, in the right iliac region. As the caecum, or caput coli, is developed, it presents 

 a conical appendage, which is at first fully as large as the small intestine and is relatively 

 longer than in the adult. During the fourth week, this appendage becomes relatively smaller 

 and more or less twisted, forming the appendix vermiformis. At the second month, the 

 caecum, or caput coli, as we have seen, is at the umbilicus, and the large intestine extends 

 in a straight line toward the anus ; at the third month, it is situated at about the middle 

 of the abdomen ; and it gradually descends, until it reaches the right iliac region at about 

 the seventh month. Thus, at the second month, there is only a descending colon ; the 

 transverse colon is formed at the third month ; and the ascending colon, at the fifth 

 month. The ileo-caecal valve appears at the third month ; the rectum, at the fourth 

 month; and the sigmoid flexure of the colon, at the fifth month. During this time, the 

 large intestine increases more rapidly in diameter than the small intestine, while the latter 

 develops more rapidly in its length. 



In the early stages of development, the surface of the intestines is smooth ; but villi 

 appear upon its mucous membrane during the latter half of intra-uterine existence. 

 These are found at first both in the large and the small intestine. At the fourth month, 

 they become shorter and less numerous in the large intestine, and they are lost at about 

 the eighth month, when the projections which bound the sacculi of this portion of the 

 intestinal canal make their appearance. The valvulae conniventes appear, in the form of 

 slightly-elevated, transverse folds, in the upper portion of the small intestine. The villi 

 of the small intestine are permanent. 



