CHAPTER XXXII 



ANIMAL ANOMALIES 



It is rather generally understood among students of biology that 

 no two animals, even of the same species, are exactly counterparts of 

 each other. There is a certain normal range of variation in size and 

 structure as well as in functional efficiency. Any variations beyond 

 these rather fixed limits are classified as malformations or anomalies. 

 A study of such abnormalities is know-n as teratology. Abnormalities 

 may occur at almost any stage in the life of the individual, but a 

 large majority result from abnormalities in the process of develop- 

 ment. Many are due to failure of development and some to over- 

 development. In turn, much of this is due to malregulation resulting 

 from failure of balance in the functioning of the endocrine system. 

 The occurrence of striking malformations in newborn human babies is 

 in approximately the ratio of one to 165. Fortunately, many of the 

 most grotesque anomalies do not reach full development and are bom 

 dead. 



Anomalies have been classified into several groups on the basis of 

 the following conditions : 



a. Failure of Development. — The embryonic structure fails to ap- 

 pear or at least fails to develop to a significant degree. Examples 

 of this account for single kidneys or legs where they are usually 

 paired. 



b. Arrested Development in which the development stops before 

 adult condition is reached, as in the cleft palate, harelip, or dia- 

 phragmatic hernia. 



c. Overdevelopment.— In such cases growth is exaggerated or the 

 number of parts increased as in gigantism (macrosomia) or in- 

 creased number of digits or accessory mammary nipples. 



d. Fusion. — The kidneys are sometimes fused together to form 

 a horseshoe kidney. 



e. Splitting, thus forming extra structures, as in the case of acces- 

 sory spleens or the splitting of the ureter. 



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