ANIMAL ANOMALIES 



655 



f. Displacement of Organs. — Cases are found where organs oc- 

 cupy abnormal positions, as both kidneys on the same side, a finger 

 at the wrist, or the appendix on the left side. 



Causes of Anomalies. — There are both internal and external 

 agencies which bring about malformations in the individual. Both 

 embryology and pathology contribute to the explanation of the 

 causes of these abnormal conditions. The development may be per- 

 fectly normal and a subsequent disease may be the cause of striking 

 abnormality. On the other hand, certain diseases of the parent will 

 influence the normal development of the fetus. Even twinning by 

 itself is an abnormal process in most animals. It, however, is not 

 usually thought of under the title of anomaly, except when they are 



Fig. 354. — Grasshoppers at time of diapause, showing some of the abnormahties 

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

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

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

 double abdomen. (From Evans, "Effects of Roentgen Radiation on Embryonic 

 Organization and Regulation in Melanoplus differentiaUs [Orthoptera]," Physio- 

 logical Zoology, Vol. X.) 



physically connected or the individuals are otherwise malformed. 

 The causes of anomalies may be summarized as follows : 



1. Internal Causes. 



a. The germ plasm carries hereditary causes for some, and these 

 characteristics are transmitted as are normal traits. Color blind- 

 ness and hemophilia (bleeder condition) are examples. 



b. Diseases which cause abnormal growths and conditions, as 

 elephantiasis. (Fig. 389.) 



c. Unbalancing the chemical regulators (hormones) which are pro- 

 duced by the endocrine glands. (Overactivity of the hypophysis 

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

 physically connected or the individuals are otherwise malformed, 

 in thyroid activity.) 



