394 ON THE NUMBER OF POLAR BODIES AND [VI. 



fishes. But several embryos and young animals are not 

 developed in this way, for embryonic development soon ceases, 

 and the egg dies. 



The recent observations of Born ^ upon the eggs of the frog 

 also make it very probable that a double development is pro- 

 duced by the entrance of two spermatozoa into the ^g%, but 

 here also only monstrosities, and not twins, were produced. 

 On the other hand, it has been shown that in birds twins may 

 be produced from the same egg, and there is no reason for the 

 belief that their production is due to multiple impregnation. 

 But if it may be assumed that human twins, when identical, 

 have been derived from a single egg, it seems to me to be 

 extremely probable that fertilization was also eifected by a 

 single sperm-cell. We cannot understand how such a high 

 degree of similarity could have been produced if two sperm- 

 cells had been made use of, for we are compelled to assume 

 that two such cells would very rarely contain identical germ- 

 plasms. 



It is most probable that the egg-nucleus coalesces with the 

 nucleus of a single spermatozoon, but the resulting segmenta- 

 tion-nucleus divides together with the cell-body itself, without 

 the occurrence of those ontogenetic changes in the germ- 

 plasm which normally take place. The nucleoplasm of the 

 two daughter-cells still remains in the condition of germ-plasm, 

 and its ontogenetic transformation begins afterwards — a trans- 

 formation which must of course proceed in the same way in 

 both cells, and must lead to the production of identical off- 

 spring. This is at least a possible explanation which we may 

 retain until it has been either confirmed or disproved by fresh 

 observations, — an explanation which is moreover supported 

 by the well-known process of budding in the eggs of lower 

 animals. 



VI. Recapitulation. 



To bring together shortly the results of this essay : — the 

 fundamental fact upon which everything else is founded is the 

 fact that two polar bodies are expelled, as a preparation for 

 embryonic development, from all animal eggs which require 



* Born, ' Uebcr Doppclbildiingen beim Frosch und deren Entstehung.' 

 Brcslauer arztl. Zeitschrift, 1882, 



