g 2 8 EMBRYOLOGY 



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 intes- 

 tine are permanent. 



The mesentery is first formed of two perpendicular folds attached to 

 the sides of the spinal column. As the intestine undergoes develop- 

 ment, a portion of the peritoneal membrane extends in a quadruple fold 

 from the stomach to the colon, to form the great omentum, which covers 

 the small intestine in front. 



As the head undergoes development a large cavity appears, which 

 finally is bounded by the arches that are destined to form the different 

 parts of the face. This is the pharynx. It is entirely independent, in 

 its formation, of the intestinal canal, the latter terminating in a blind 

 extremity at the stomach ; and between the pharynx and the stomach, 

 there is at first no channel of communication. The anterior portion of 

 the pharynx presents, during the sixth week, a large opening, which is 

 afterward partially closed in the formation of the face. The rest of the 

 cavity remains closed until a communication is effected with the 

 oesophagus. The oesophagus appears in the form of a tube, which 

 finally opens into the pharynx above and into the stomach below. At 

 this time there is really no thoracic cavity, the upper part of the 

 stomach is very near the pharynx, the oesophagus is short, the rudi- 

 mentary lungs appear by its sides and the heart lies just in front. As 

 the thorax is developed, however, the oesophagus becomes longer, the 

 lungs increase in size, and finally the diaphragm shuts off its cavity 

 from the cavity of the abdomen. The growth of the diaphragm is from 

 its periphery to the central portion, which latter gives passage to the 

 vessels and the oesophagus. 



The development of the anus is very simple. At first the intestine 

 terminates below in a blind extremity ; but at about the seventh week 

 a longitudinal slit appears below the external organs of generation, by 

 which the rectum opens. This is the anus. The opening of the anus 

 appears about a week after the opening of the mouth, at or about the 

 seventh week. 



The rudiments of the liver appear very early, and, indeed, at the 

 end of the first month this organ has attained a large size. Two pro- 

 jections, or buds, appear on either side of the intestine, which form the 

 two principal lobes of the liver. This organ is at first symmetrical, the 

 two lobes being of nearly the same size, with a median fissure. One of 

 these prolongations from the intestine becomes perforated and forms 

 the excretory duct, of which the gall-bladder, with its duct, is an append- 

 age. During the early part of foetal life the liver occupies the greatest 



