622 TEXT-BOOK OF EMBRYOLOGY. 



pharynx, that is, within the sides of the neck. This abnormal process results in 

 various forms of cysts and tumors. The most common are simple retention 

 cysts, known as branchial or branchio genetic cysts, which vary from small 

 insignificant structures to large tumors. If derived from the external branchial 

 furrows, they are dermoid in character, lined with ectodermal derivatives, and 

 contain sebaceous material. If derived from the internal furrows, they con- 

 tain mucous fluid, the lining epithelium is likely to be columnar and is claimed 

 by some to be ciliated. 



DEFECTS IN THE THORACIC AND ABDOMINAL REGIONS, AND THEIR ORIGIN. 



As described elsewhere (see page 317), the digestive tube (primitive gut) and 

 ventral body wall are formed primarily by a bending ventrally and fusing of the 

 originally flat germ layers. The splanchnopleure on each side first bends 

 ventrally and fuses with its fellow of the opposite side in the medial line to form 

 the gut, and soon afterward the somatopleure likewise fuses in the ventro- 

 medial line to form the body wall. Naturally a defective fusion of the two 

 sides of the somatopleure would result in a more or less extensive medial cleft. 

 The cleft may be limited to a small portion of the abdomen or thorax, or may 

 extend from the neck to the pelvis. 



When the cleft is very extensive and involves the thoracic and abdominal 

 walls, the condition is known as thoracogastroschisis. In this case most of the 

 viscera protrude through the cleft (ectopia viscerum) and are covered merely 

 by peritoneum. Spinal curvature of a low or high degree is usually associated 

 with the eventration. 



The cleft may involve the entire abdominal wall gastroschisis completa 

 and the abdominal viscera may protrude through it. In a somewhat lesser 

 degree of fission, parts of trre abdominal viscera, covered with peritoneum, 

 may protrude and form what is known as omphalocele. Not uncommonly 

 portions of the intestine and omentum protrude through an abnormally large 

 umbilical ring umbilical hernia. The region below the umbilicus is not in- 

 frequently the seat of fissures in the abdominal wall, through which the bladder 

 may protrude (ectopia vesicae). Fissures in the thoracic wall vary in extent. 

 When the defect is extensive the heart and pericardium protrude through the 

 opening (ectopia cordis). 



MALFORMATIONS OF THE EXTREMITIES. 



Any degree of deficiency may exist, from total absence of extremities to 

 the lack of a single finger. The malformations, however, are not confined to 

 total or partial lack of members, for supernumerary fingers and toes are some- 

 times present. The following is the classification given by Piersol : 



