166 



(5) Monsters tbrough fusion of parts— one central eye, one nasal 

 opening, etc. 



(6) Monsters through abnormal position or form of parts — curved 

 spine, face, limb, etc. 



(7) Monsters through excess of formation — enormous head, super- 

 numerary 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 mon- 

 strosities, like extra digits, absence of horns or tails, etc., run in fami- 

 lies and are produced almost as certainly as color or form. Others are 

 associated with too close breeding, the powers of symmetrical develop- 

 ment beiug interfered with, just as in other cases a sexual incompati- 

 bility is developed, near relatives failing to breed with each other. 

 Mere arrest of development of a part may arise from accidental 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 simultaneously 

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

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

 of development, this factor can operate to far greater purpose 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 attached 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 disturbing conditions in giving such wrong directions to the 

 developmental forces is well shown in the experiments of St. Hilaire 

 and Valentine in varnishing, shaking, and otherwise breaking up the 

 natural connections in eggs, and thereby determining the formation of 

 monstrosities at will. So, in the mammal, blows and other injuries 

 that detach the foetal membranes from the walls of the womb or that 

 modify their circulation by inducing inflammation are at times followed 

 by the development of a monster. The excitement, mental and phys- 

 ical, attendant on fright occasionally acts in a similar way, acting prob- 

 ably through the same channels. 



