326 Vertebrate Embryology 



50 cm. long at birth, may be reduced to 10 cm., 

 making parturition difficult or impossible ; or 

 it may reach a length of 2 to 3 meters, in 

 which case its coils- may cause trouble by pro- 

 ducing knots to impede circulation. Various 

 abnormalities of the chorion and placenta also 

 occur. 



Abnormalities of the foetus proper will be 

 described in two groups : first, monstra per 

 defectum ; and second, monstra per excessum. 



Monstra per defectum. Abnormalities of 

 this class may be grouped in two divisions : 

 A, simple anomalies ; B, abnormalities of ar- 

 rested development. As examples of class A 

 may be mentioned : amorphous embryos, shape- 

 less, skin-covered masses ; acephalous, or head- 

 less embryos ; microcephalic embryos, or those 

 with very small heads ; cyclops, or one-eyed 

 embryos ; embryos with one or more extremi- 

 ties or parts of extremities missing ; embryos 

 with ribs, vertebrae, or, in fact, almost any 

 other part of the body missing. 



The abnormalities of class B, arrested de- 

 velopment, are among the most interesting of 

 human anomalies. Among these may be men- 

 tioned : double uterus, caused by the incomplete 

 fusion of the Miillerian ducts, the normal con- 



