328 Vertebrate Embryology 



stincts of the male. Other examples of arrested 

 development might be given, but the above 

 will give an idea of what is meant by that class 

 of abnormalities. 



Monstra per excessum. Abnormalities of 

 this class also may be arranged in two groups : 

 A, over-large development, as in giants, where 

 the whole body is abnormally large ; mega- 

 cephalic individuals, with very large heads ; 

 individuals with any other part of the body 

 abnormally enlarged. B, supernumerary for- 

 mation, as in twin monsters, joined in all con- 

 ceivable ways ; supernumerary mammae, fingers, 

 or almost any other part of the body. 



Before leaving the subject of abnormalities 

 it may be well to say a few words as to the 

 supposed cause of multiple births, of both nor- 

 mal and abnormal infants. The occurrence 

 of twins is, of course, quite common, and trip- 

 lets and quadruplets are not unknown. Cases 

 of six or even eight embryos in one uterus 

 have been reported, but they are of very doubt- 

 ful authenticity. 



In the case of double births we may have 

 either fraternal or duplicate twins, or double 

 monsters. Fraternal twins are usually no more 

 alike than any two children of the same parents; 



