382 ON THE NUMBER OF POLAR BODIES AND 



in which differences have been exhibited by twins which were en- 

 closed in the same embryonic membranes ; but nevertheless I be- 

 lieve that two spermatozoa are not necessary to cause the formation 

 of twins by a single egg. We know, it is true, from the investi- 

 gations of Fol 1 , that multiple impregnation produces the simul- 

 taneous beginning of several embryos in the eggs of star-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 2 upon the eggs of the frog also 

 make it very probable that a double development is produced by 

 the entrance of two spermatozoa into the egg, 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 effected 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 segmentation- 

 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 transformation which must of 

 course proceed in the same way in both cells, and must lead to the 

 production of identical offspring. This is at least a possible ex- 

 planation which we may retain until it has been either confirmed 

 or disproved by fresh observations, an explanation which is more- 

 over supported by the well-known process of budding in the eggs 

 of lower animals. 



1 Fol , ' Recherches sur la f^condation et le commencement de l'he"nogenie.' Geneve, 

 Bale, Lyon. 1879. 



a Born, ' Ueber Doppelbildungen beim Frosch und deren Entstehung.' Breslauer 

 arztl. Zeitschrift, 1882. 



