226 BIOLOGY: GENERAL AND MEDICAL 



seen in fish, then to that in the batrachians, then to 

 that of mammals as can at once be seen by comparing 

 the figures showing the embryology of the human heart 

 and vessels with those of the different phylogenetic 

 groups. 



It has been shown that the first separation of the 

 egg into two blastomeres is accompanied by a fair 

 uniformity in the developmental power of each, so that 

 if separated at that time, two embryos of small size 

 may develop. Accidental separation of the primary 

 blastomeres seems at times to occur among the highest 

 animals, and it is probably in this manner that homol- 

 ogous twins arise. Such twins are always of the same 

 sex; resemble each other so closely that throughout life 

 they are frequently mistaken one for the other; possess 

 the same general tendencies of mind and body; are 

 predisposed to the same diseases; attain to about the 

 same general intellectual development, and not infre- 

 quently die within a short time of each other, some- 

 times from the same cause. Galton's studies of homolo- 

 gous twins, in his book upon " The Human Faculty, " are 

 most interesting as showing how completely the homol- 

 ogous twins are identified. 



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 



