548 VERTEBRATE LIFE AND ORGANIZATION 



Twins have been produced experimentally by the first method in lower 

 vertebrates. This method is a possibility in mammals, but it is not as 

 likely to occur as the others, for the mammalian egg and cleavage stages 

 are surrounded by a strong membrane, the zona pellucida, that should 

 prevent the blastomeres from separating (Fig. 31.1). 



A particularly interesting case of twinning is seen in the armadillo. 

 This animal always has quadruplets and the four individuals are always 

 of the same sex. The fact that only one corpus luteum is found in the 

 ovary, which means that only one follicle and egg matured and ovu- 

 lated, indicates that all are identical twins. When the blastocyst is ex- 

 amined, it is discovered that the inner cell mass has subdivided into 

 foia- parts. 



If identical twins are produced by subdivision of the inner cell mass, 

 or by the formation of two primitive streaks, one would expect to find 

 occasional cases in which the separation is incomplete. Though fortu- 

 nately rare, conjoined twins are born from time to time. All degrees of 

 union have been found. Usually such individuals die in infancy, but the 

 most famous pair, Chang and Eng, lived to be 63. Though Chinese, 

 Chang and Eng were born in Siam. They worked for a circus, married 

 and fathered 22 children! Their fame led to the popular term "Siamese 

 twins" for such conjoined twins. 



Questions 



1. How has the early development of mammals been modified by the retention of the 

 embryo in a uterus? 



2. Compare endoderm and mesoderm formation in a mammal and a frog. 



3. To what extent does ontogeny recapitulate phylogeny? 



4. How does it happen that certain parts of the digestive tract are of ectodermal origin? 



5. What structures develop from the somites, the nephrogenic ridge and the lateral plate 

 mesoderm? 



6. What sort of changes occur in the human fetus after the second month? 



7. Distinguish between fraternal and identical twins. How may identical twins be formed? 



Supplementary Reading 



Corner has written a very interesting essay on human development entitled Ourselves 

 Unborn. Further information on the development of man and other vertebrates can be 

 found in such standard texts as Arey, Developmental Anatomy, or Witschi, Develop- 

 ment of Vertebrates. Those interested in experimental embryology will find Willier, Weiss 

 and Hamburger, Analysis of Development, an invaluable source. The relationship of 

 embryology to evolution is carefully discussed by DeBeer in Embryos and Ancestors. The 

 biology of twinning and the differences and similarities between fraternal and identical 

 twins are thoroughly considered by Newman, Freeman and Holzinger, Twins. 



