238 BIOLOGY: GENERAL AND MEDICAL 



human embryo (twenty-five days) and gives it a striking 

 resemblance to the embryos of lower animals. 



The heart makes its appearance as a simple straight 

 tube similar to that of the invertebrates, becomes 

 separated into an anterior ventricle and posterior auricle 

 like that of the fishes. Later it consists of a curved 

 organ with two incompletely separated auricles and one 

 ventricle, not unlike that of the batrachians. Much 

 later it differentiates into the four-chambered viscus of 

 the higher vertebrates. 



Not only does the development of the heart thus 

 conform to its phylogeny, but the development of the 

 whole circulatory system coincides as well. The heart 

 and great vessels first appear, the small vessels and 

 capillaries later. Further, the arrangement of the 

 arteries at first conforms with fair accuracy to that 

 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. 



