676 REPRODUCTION. 



sion, form the bronchial tubes and their ramifications. At first there 

 is no distinction between the chest and abdomen, and the lungs conse- 

 quently project into the upper part of the abdominal cavity. Afterward, 

 an outgrowth takes place on each side, in the form of a transverse par- 

 tition, which gradually closes together and becomes the diaphragm, 

 thus shutting off the cavity of the chest from that of the abdomen. 

 Before the closure of the diaphragm, 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, 

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

 tially protrude into the chest, forming congenital diaphragmatic her- 

 nia. The lung on the affected side usually remains in a state of 

 imperfect development. Diaphragmatic hernia of this character is more 

 frequently found on the left side than on the right, and may sometimes 

 continue in adult life without causing serious inconvenience. 



Urinary Bladder and Urethra. The urinary bladder originates 

 from an outgrowth of the primitive intestine which at first appears 

 as the allantois (page 641). In the lower animals this outgrowth 

 retains everywhere the character of a hollow sac. In man, that por- 

 tion which is situated in the body of the embryo and its immediate 

 vicinity is also hollow ; while beyond this point it spreads out in the 

 form of a single continuous investing membrane the " choriori." Its 

 elongated portion, between the chorion and the abdomen of the foetus, 

 is the " umbilical cord," which at first contains a central tubular canal 

 throughout most of its length. This canal becomes subsequently ob- 

 literated ; the obliteration commencing at its outer extremity and thence 

 extending inward to the umbilicus. Inside the umbilicus it still pro- 

 ceeds for a certain distance and then ceases. Thus the original protru- 

 sion of the intestinal canal within the abdomen, which gave rise to the 

 allantois and chorion, is divided into two portions. The first portion, 

 immediately connected with the intestine, remains hollow, and after- 

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

 urinary bladder and the umbilicus, is consolidated into a rounded cord, 

 termed the urachus. 



The urinary bladder is at first a pyriform sac (Fig. 224, e), commu- 

 nicating at its base with the lower portion of the intestine, and con- 

 tinuous by its pointed extremity with the urachus, by which it is 

 attached to the abdominal walls at the umbilicus. Afterward, the blad- 

 der loses this conical figure, and its superior fundus assumes in the 

 adult a rounded form. 



Development of the Mouth and Face. The alimentary canal is at 

 first a cylindrical tube, closed at both extremities. In the region of the 

 abdomen, which in the earlier periods of development occupies nearly 

 the whole of the body, the mesoderm separates, as previously described 

 (page 635), into two laminae, an outer and an inner. The outer lamina, 

 consisting of the external integument and the subjacent voluntary mus- 

 cles, forms the parietes of the abdomen. The inner lamina forms the 



