ANIMAL ANOMALIES 



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c. Unbalancing the chemical regulators (hormones) which are 

 produced by the endocrine glands. (Overactivity of the hypophysis 

 causes gigantism; cretinism, a dwarf condition, results from deficiency 

 in thyroid activity.) 



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 connections 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. 



Fig, 67. — Grasshoppers at time of diapause, showing some of the abnormalities 

 which very infrequently occur in their natural development. 1, Normal embryo : 

 2, embryo with two extra heads and mouth parts ; 3, embryo with lateral twin 

 joined at the abdomen ; ^, almost complete twins back to back ; 5, embryo with a 

 double abdomen. (From Evans, Effects of Roentgen Radiation, Physiol. Zool., Vol. 

 X.) 



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

 may cause some abnormalities. 



c. Abnormal 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 inhibition or arrest brought about by deficiencies 

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



