MALFORMATIONS. 93 



tion, infectious diseases, etc. External causes exert their in- 

 fluence during the embryonic or formative period and they 

 must act in a mild degree or death of the embrvo and abor- 

 tion follows. 



Typical malformations are approximately of the same form 

 and are usually produced by similar causes. Atypical mal- 

 formations are variable in form and may be produced bv a 

 variety of causes. 



A complete description of all malformations is bevond the 

 scope of general pathology. A general classification with a 

 description and origin of the most striking malformations is 

 all that will be attempted in this chapter. 



Alalformations may be divided into two classes; 1st, Single 

 malformations, and 2nd, Double or multiple malformations. 



Single malformations are tliose affecting a single individual. 

 Single malformations may be groujjcd into five classes as fol- 

 lows: Malformations restdting from; (a) arrested growth or 

 development; (b) excessive growth or development; (c) trans- 

 position of visceral organs; (d) persistent foetal structures; (e) 

 mixture of sexual organs. 



.\ERi-:STKD DEVELOPMENT. — Alal formations caused by arrested 

 development may involve an entire individual or any part ot 

 an individual. Arrested development of the entire individual 

 results in the formation of an irregular, fleshy mass, called a 

 mole, in place of the normal foetus. ?\loles may be carried in 

 the uterus for the entire period of gestation. In some instances 

 a mole and a normal foetus may 1)e delivered at the same time. 

 Moles have been observed in mares, more rarely in cows. 



^lalformations resulting from arrested development of a part 

 may be manifested bv the entire absence of the part (aplasia), 

 bv underdevelopment of the part, (hypoplasia), or by a lack 

 of union or fusion of tissue fschistosis and atresia). The fol- 

 lowing malformations are the result of local aplasia. 



Accphahis. A name applied to a headless monstrosity. 

 Acephalus is probably the result of amniotic adhesions. 



Atrichia. A defect in which there is no hair. This results 

 from some disturbance of cutaneous development. 



Ann'clus. A malformation in which the spinal cord is absent. 

 Defects of the primitive streak or failure of production of the 

 neural canal interferes with or prohibits the formation of the 

 spinal cord and is the cause of amyelus. 



Acardia. A heartless monstrosity. 



Agastric. A malformation in which the afifected i.ndividua' 



