178 HEREDITY AND EUGENICS 



necessar}?- to produce polyenibryony.* In the deer 

 it is not known when the arrest occurs, but failure of 

 budding (twins) may be due to (i) arrest being at 

 the wrong stage of development, or (2) lack of a 

 tendency" for the embr3^o to bud. Possibly delayed 

 implantation of the blastocyst, which may in turn 

 be controlled by the condition of the corpora lutea, 

 may account for the production of identical twins in 

 man. A peculiarity^ in the activity of the corpora 

 lutea would be as likely to be inherited as are 

 differences in other glands of internal secretion, and 

 such a difference would then become a factor in the 

 inheritance of twinning. 



Newman, who has investigated twinning for a 

 number of 3^ears, published a little book on the 

 subject (191 7), and has recently (1921) taken up 

 experiments on the production of twin embryos. 

 He used a Californian starfish, and studied the 

 twinning which may occur in spontaneous^ partheno- 

 genetic larvae and in hybrids with another species. 

 He looks upon twinning as a process which involves 

 the duplication of originally single structures. The 

 first step involved is retarded development, followed 

 by loss of organisation or dedifferentiation. Re- 

 cover}^ results in the formation of new apical points, 

 which form the anterior ends of new individuals. In 

 the starfish Patiria such arrested development may 

 result in physiological isolation of blastomeres in 



* Regarding the interpretation of polyembryony in the 

 armadillo, Professor MacBride suggests that all the South Ameri- 

 can Edentates are originally descended from tree-living animals, 

 in which it is an advantage to have only one young at a birth, 

 and a dome-shaped placenta as in the apes. The armadillos 

 have taken to the ground where the risks are greater, but have not 

 been able to recover the habit of laying more than one egg. They 

 have, however, substituted the method of multiplication by 

 budding from the single egg. The fact that in man also identical 

 twins occur may similarly be a result of his arboreal ancestry. 



