384 A TREATISE ON HORSE-BREEDING. 



(7) Monsters through excess of formation enormous head, 

 supernumerary digits, etc. 



(8) Monsters through imperfect differentiation of sexual 

 organs hermaphrodites. * 



(9) Double monsters double-headed, double-bodied, extra 

 limbs, etc. ' 



The causes of monstrosities appear to be very varied. 

 Some monstrosities, like extra digits, absence of horns or 

 tails, etc., run in families and are produced almost as cer- 

 tainly as color or form. Others are associated with too close 

 breeding, the powers of symmetrical development being inter- 

 fered with, just as in other cases a sexual incompatibility is 

 developed, near relatives failing to breed with each other. 

 Mere arrest of development of a part may arise from acci- 

 dental disease of the embryo; hence vital organs are left out, 

 or portions of organs, like the dividing walls of the heart, 

 are omitted. Sometimes an older foetus is inclosed in the 

 body of another, each having started independently from a 

 separate ovum, but the one having become embedded in the 

 semi-fluid mass of the other and having developed there sim- 

 ultaneously with it, but not so largely nor perfectly. In 

 many cases of redundance of parts the extra part or member 

 has manifestly developed from the same ovum and nutrient 

 center with the normal member to which it remains adher- 

 ent, just as a new tail will grow out in a newt when the former 

 has been cut off. In the early embryo, with its great powers 

 of development, this factor can operate to far greater pur- 

 pose than in the adult animal.. Its influence is seen in the 

 fact pointed out by St. Hilaire that such redundant parts are 

 nearly always connected with the corresponding portions in 

 the normal foetus. Thus superfluous legs or digits are at- 

 tached to the normal ones, double heads or tails are con- 

 nected to a common neck or rump, and double bodies are 

 attached to each other by corresponding points, navel to 

 navel, breast to breast, back to back. All this suggests the 

 development of extra parts from the same primary layer of 

 the impregnated and developing ovum. The effect of dis- 

 turbing conditions in giving such wrong directions to the 

 developmental forces is well shown in the experiments of St. 



