ONTOGENESIS 239 



As such twin embryos are, during their embryonal 

 development, in close proximity to one another, there 

 seems to be an occasional tendency for the growing cells 

 of one embryo to become confused with those of its 

 neighbor, with interesting resulting malformations. 

 Thus, one-half may grow rapidly, outstrip and include 

 the other, whose growth is consequently disturbed and 

 inhibited, so that one foetus with normal external con- 

 figuration, but with internal confusion explainable on no 

 other hypothesis, may arise. 



Or the foetuses may be equal or nearly equal in size, 

 but blended or connected throughout, or at the cephalic, 

 thoracic, or pelvic portions, sometimes face to face, some- 

 times side to side, sometimes back to back. The rela- 

 tive position of the conjoined parts 'is usually normal 

 i.e., the cephalic and caudal ends of the embryos 

 correspond. Sometimes, however, they rotate until 

 the cephalic ends are opposed and the caudal ends 

 conjoined, or the caudal ends opposed and the cephalic 

 ends conjoined. 



In rare cases the relation of one embryonal axis to 

 the other is lost and the attachment of one embryo 

 to the other without correspondence of the parts. 



The attachment of one individual to the other may 

 embrace the fundamental and vital organs heart, 

 brain, spinal cord, etc.; or may be through compara- 

 tively unimportant structures, the twins being conjoined 

 by a kind of slender pedicle, as in the Siamese twins. 



Some knowledge of embryology likewise enables one 

 to understand certain monstrous formations arising in 

 single individuals. Thus the bridging of the neural 

 canal to form the spinal canal in which the future spinal 

 cord is to lie, is one of the first features of mammalian 

 development. If this process fail anteriorly, the im- 

 perfect covering permits morbid changes, known as 

 craniorrachischisis, with meningocele or encephalomen- 

 ingocele, and terminating in anencephaly; when occur- 

 ring posteriorly, in my elo meningocele and spina bifida. 



