656 TEXTBOOK OF ZOOLOGY 



d. Fortuitous abnormalities whose causes are not apparent or are 

 influenced by certain variations in other organs. Such anomalies 

 show up most frequently in the circulatory and nervous systems. 

 Venous or nervous supplies to organs are often modified. Another 

 example is the rearrangement brought about by diaphragmatic hernia 



2. External Causes. 



a. Environmental agencies may affect almost any individual whose 

 development occurs outside the body of its parent. Exposure to 

 radium or x-ray radiation, sharp variations in temperature, exces- 

 sive salt content, or contact with toxic substances may all be respon- 

 sible for various degrees of abnormality. The same factors are also 

 effective, if present, in the uterine environment of the placental 

 type of animal. 



b. Mechanical factors, such as abnormal pressure, blows, and falls, 

 may cause some abnormalities. 



e. Ahnormal implantation in the wall of the uterus resulting in 

 deficiencies in nourishment and support of the fetus. 



d. Such diseases as syphilis, which may be transmitted from mother 

 to offspring, are responsible for some types of defects, as impaired 

 vision, 



e. Developmental inhihition or arrest brought about by deficiencies 

 in metabolism at a time when the rate or efficiency should be high. 

 The work of several embryologists seems to indicate that the pro- 

 duction of twins either as normal individuals or otherwise is re- 

 lated to this condition. 



Harelip and Cleft Palate 



These two defects are related and are sometimes found together. 

 The lateral palatine processes may fail to complete growth and 

 unite properly, thus leaving a gap in the roof of the mouth which 

 opens directly into the nasopharynx above. 



Harelip (cheiloschisis) is a very common defect and is due to the 

 failure of union of the nasal and maxillary processes. There may 

 be double harelip or single harelip. In connection with double hare- 

 lip it sometimes happens that the premaxilla projects beyond the 

 outline of the face to form what is called wolf-snout. Harelips are 

 frequently remedied by a surgical operation early in life. 



