TERATOGENESIS. 623 



1. One or More Extremities Wanting. (a) Amelus. Both upper and lower 

 extremities are practically absent, (b) Abrachius, Apus. Either the upper or 

 lower extremities are wanting, the other pair often being well formed, (c) 

 Monobrachius, Monopus. One upper or one lower extremity is absent, the 

 others being fully developed. 



2. One or More Extremities Defective. (a) Peromelus. All the extremities 

 are imperfect. A striking variety of this is the suppression of the proximal 

 segments of the extremities, the hands and feet being fairly well formed (phoco- 

 melus). (b) Perobrachius, Per opus. The former signifies defective develop- 

 ment of the upper, the latter of the lower extremities. 



3. One or More Extremities Abnormally Small but well Formed. (a) Micro- 

 melus. All the extremities are diminutive, but without any other malformation. 

 (b) Microbrachius, Micropus. One or both upper extremities may be small, or 

 one or both lower. 



4. Bones Defective or Absent. Such malformations are rare. 



5. Lower Extremities Fused. (a) Sympus (symelus siren). The lower ex- 

 tremities are fused more or less completely, and the lower end of the trunk is 

 abnormal. The feet may be imperfect and double (sympus dipus), or a single 

 foot may be present (sympus monopus), or the feet may be wanting (sympus 

 apus) . 



6. Hands or Feet Defective. There is a great variety of malformations of the 

 hands and feet due to arrested development of some of the digits. The varia- 

 tions include all degrees of suppression from the shortening of a finger or toe to 

 almost total absence of digits. Fusions of two or more digits are not uncom- 

 mon. Occasionally a structure, suggesting the webs on the feet of some 

 aquatic animals, is present. 



The condition known as polydactyly (an increase over the normal number 

 of digits) is occasionally met with. The increase may range from a partial 

 doubling of the distal segment of a finger or toe to a two-fold quota of digits. 

 Cases of ten digits are extremely rare, as are even cases of seven or eight. One 

 supernumerary finger or toe, rudimentary or complete, is not uncommon. The 

 extra digits may appear on a single hand or foot, or on both hands or feet, or on 

 all four extremities, not necessarily showing any symmetry. A statement of the 

 possible modes of origin of polydactyly will be found on page 217. 



AMNIOTIC ADHESIONS. Many malformations affecting the embryo or foetus 

 have been included under this head. It has been generally thought that the 

 amnion might become attached to some part of the embryo in such a way as 

 to cause malformations by interfering with the normal processes of growth. 

 The amnion might become attached to the head and by interfering with 

 normal growth produce hare lip, facial clefts and generally serious disturb- 

 ances. Undue pressure on an extremity would cause it to be stunted, or 

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