AND ITS APPENDAGES. 779 



the urine and the amniotic fluid ; while the liver begins to produce the 

 saccharine substance which it contains after birth. 



Lungs, Thoracic Cavity, and Diaphragm. The anterior portion of 

 the alimentary canal, which occupies the region of the neck, is the 

 oesophagus. It is straight, and, at first, very short ; but it subsequently 

 increases in length, simultaneously with the growth of the neighboring 

 parts. As the oesophagus lengthens, the lungs begin to be developed 

 by a protrusion from the anterior portion of the oesophagus, represent- 

 ing the commencement of the trachea. This protrusion soon divides 

 into two symmetrical branches, which themselves elongate and become 

 repeatedly subdivided, forming the bronchial tubes and their ramifica- 

 tions. At first, the lungs project into the upper part of the abdominal 

 cavity ; for there is still no distinction between the chest and abdomen. 

 Afterward, a horizontal partition begins to form on each side, at the 

 level of the base of the lungs, which gradually closes together to form 

 the diaphragm, and which finally shuts off the cavity of the chest from 

 that of the abdomen. Before the closure of the diaphragm is com- 

 plete, an opening exists by which the peritoneal and pleural cavities 

 communicate with each other. In some instances the development of 

 the diaphragm is arrested at this point, either on one side or the other, 

 and the opening remains permanent. The abdominal organs then par- 

 tially protrude into the cavity of the chest on that side, forming con- 

 genital diaphragmatic hernia. The lung on the affected side usually 

 remains in a state of imperfect development. Diaphragmatic hernia of 

 this character is more frequently found upon the left side than upon 

 the right, and may sometimes continue until adult life without causing 

 serious inconvenience. 



Urinary Bladder and Urethra. Soon after the formation of the 

 intestine a vascular outgrowth takes place from its posterior portion, 

 which gradually protrudes from the open walls of the abdomen, until it 

 comes in contact with the external investing membrane of the egg 

 (Fig. 287, f)] forming subsequently, by its continued growth and ex- 

 pansion, the allantois in the lower animals, the chorion in man. 



The chorion, in the portion immediately connected with the body of 

 the embryo, has, like the allantois, the form of a hollow canal ; but as 

 it spreads out, to constitute the external investment of the egg, it takes 

 the shape of a continuous membrane, forming the chorion proper 

 (p. 746). The tubular cavity of its connecting portion, the umbilical 

 cord, subsequently becomes obliterated ; the obliteration commencing 

 at its outer extremity and gradually proceeding inward until it reaches 

 the umbilicus. Inside the umbilicus it still proceeds for a certain dis- 

 tance and then ceases. Thus the original protrusion of the intestinal 

 canal within the abdomen, which gave rise to the allantois and the cho- 

 rion, is divided into two portions. The first portion, or that imme- 

 diately connected with the intestine, remains hollow, and forms after- 

 ward the urinary bladder. The second portion, between the urinary 



