DIPFICXTLT PABTTJEITION. 175 



rical development being interfered with, just as in other cases a sex- 

 ual incompatibility 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 fetus is inclosed in the body of another, each 

 having started independently from a separate ovum, but the one 

 having become embedded in the semifluid mass of the other and hav- 

 ing 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 former has been cut off. 

 In the early embryo, with its great powers of development, this fac- 

 tor 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 fetus. Thus superfluous legs or digits are 

 attached to the normal ones, double heads or tails are connected to a 

 common neck or rump, and double bodies are attached to each 9ther 

 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 break- 

 ing 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 fetal 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 physical, attendant on fright occasionally acts in a simi- 

 lar way, acting probably through the same channels. 



The monstrous forms likely to interfere with parturition are such 

 as, from contracted or twisted limbs or spine, must be presented 

 double ; where supernumerary limbs, head., or body must approach the 

 passages with the natural ones; where a head or other member has 

 attained to an unnatural size ; where the body of one fetus has become 

 inclosed in or attached to another, etc. 



Extraction is sometimes possible by straightening the members and 

 securing such a presentation as will reduce the presenting mass to its 

 smallest and most wedgelike dimensions. To effect this it may be 

 needful to cut the flexor tendons of bent limbs or the muscles on the 

 side of a twisted neck or body ; and one or more of the manipulations 

 necessary to secure and bring up a missing member may be required. 



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