602 TEXT-BOOK OF EMBRYOLOGY. 



b. One body normal, the other abnormal or much mal- 

 formed: Unequal monochorionic twins. 



2. The two bodies united; formed alike (equal), or one remains 

 more or less rudimentary (unequal) : Twins joined together; 

 duplicate monsters. 



a. Union confined to lower end of body : Double monsters 

 with posterior union; anterior duplicity. 



b. Union confined to middle of body, or extending from 

 middle forward: Double monsters with middle union. 



c. Union limited to upper end of body, or extending from 

 upper end downward: Double monsters with anterior 

 union; posterior duplicity. 



B. Duplicity does not affect entire anlage, but only a part : Incomplete 

 duplicity. 



1. Two incomplete anlagen (or primitive streaks) pass over into 

 a single anlage: Posterior incomplete duplicity. 



2. An originally single anlage forms by dichotomous growth two 

 separate upper (anterior) ends: Anterior incomplete duplicity. 



In addition: Triplicity, quadruplicity, etc., (multiplicity). 

 II. Both bodies derived from two originally dissimilar, asymmetrical 

 anlagen, of which one, always rudimentary, becomes more or less en- 

 closed and nourished by the other: True parasitic duplicity; asym- 

 metrical duplicity. 



In addition: Teratoid tumors. 



SYMMETRICAL DUPLICITY. 



As seen from the foregoing scheme, there are included under this head 

 double forms in which both embryos develop within a single chorion (mono- 

 chorionic twins), and in which the bodies may be distinct and separate 

 (complete duplicity) or may be united (incomplete duplicity). In complete 

 duplicity each embryo usually possesses its own amnion and umbilical cord, 

 but both are attached to the same placenta. In such cases the two in- 

 dividuals probably developed from one fertilized ovum and thus received 

 equivalent chromatin which, with similar nutritional conditions, guided 

 the formation of equal monochorionic twins. They may grow to maturity, 

 and they always bear a remarkable resemblance to each other in physical 

 features and in mental characteristics and are always of the same sex. More 

 commonly, however, the development of separate monochorionic twins is 

 unequal, caused probably by dissimilar chromatin and nutrition. In some 

 instances the less favored individual is but slightly affected, so that it may 

 be born and be able to maintain an independent existence. On the other 



