30 DEVELOPMENT OF THE FROG'S EGG [Cii. Ill 



The eggs of the starfish can be fertilized by the spermatozoa 

 of the sea-urchin, — ^ forms much more different than any two 

 species, genera, or even families of frogs, and the early stages 

 of segmentation, and the formation of a swimming blastula and 

 gastrula may be passed through ; but the later embryonic devel- 

 opment is not carried out, and after a time the gastrulas die.^ 



Hertwig's experiments ('77) on polyspermy in the eggs of 

 echinoderms show that when several spermatozoa enter the 

 same egg a karyokinetic spindle is formed around each of the 

 resulting male pronuclei and many or all of the pronuclei 

 divide. Often the spindles are so near together that they 

 mutually influence one another and most complicated karyo- 

 kinetic figures result. Subsequently the protoplasm breaks up 

 around the pronuclei in a most irregular way, and generally 

 such eggs do not give rise to even the earliest stages of devel- 

 opment. The phenomenon is so similar to the '• barock " seg- 

 mentation of the frog's egg that it seems possible that in the 

 latter the result is brought about in the same way as in the 

 echinoderms. 



Experiments of Rauber and of Boveri 



Rauber, in 1886, tried to carry out the following interesting 

 experiment. The segmentation-nucleus of a frog's egg, one 

 hour after fertilization, was removed by means of a fine pipette. 

 The same "process was carried out with a toad's egg. The 

 nucleus of the toad's egg was then placed in the frog's egg 

 that had had its nucleus removed, and the nucleus of the 

 frog's egg was placed in the toad's egg. Unfortunately, 

 neither egg developed. The results of such an experiment 

 would be of the greatest importance if the experiment could 

 be successfully carried out ; for in this way we should hope to 

 discover whether the characters of the embryo come from the 

 nucleus or from the protoplasm of the egg. 



Boveri, in 1889, made somewhat similar experiments with 

 the egg of the sea-urchin. When the eggs are shaken in a 

 small tube, they are broken into fragments, some with nuclei 

 and others without. When a sufficiently large non-nucleated 



1 Morgan, '93, Anat. Anzeiger. 



