224 The American Naturalist. [March, 
the frog’s egg, and the nucleus of the frog’s egg into the toad’s 
egg. Now if the nucleus alone carries the hereditary functions 
then there must develop out of the frog’s egg a toad, and out of 
the toad’s egg a frog.” As was to be anticipated, the eggs did 
not develop further, and at first sight there seems to be no 
reason to expect that such an experiment should succeed. For 
if we were in a position to take out, without further injury, the 
nucleus from cells, still we could scarcely introduce by artificial 
means a new nucleus, without causing such fundamental alter- 
ations of the one or the other part, as to make further life 
impossible. 
But just here Nature herself offers the solution by means of 
which we can accomplish our purpose, because it makes the 
second and more difficult part of the experiment an entirely 
` normal process—the entrance of the spermatozoon into the egg 
may be utilized. 
The basis for my research is founded on the discovery of the 
Hertwig brothers? When these investigators shook for a long 
time the eggs of the sea-urchin in a test tube, containing a 
small amount of water, in order to alter them mechanically, 
they found that in consequence of the shaking some of the 
eggs fell into pieces, and while some of the pieces contained 
nuclei, others did not. It was further shown that these enu- 
cleated (non-nucleated) fragments, as well as the nucleated 
ones could be fertilized, and that an active segmentative pro- 
cess took place in them. What followed in these cell masses 
was not determined by the Hertwigs. I have myself verified 
this discovery during my last visit to the Zoological station 
of Naples, and found that the enucleated and fertilized fragments 
of eggs developed as far as and formed as complete larve as did 
the perfect nucleated egg. From the enucleated fragments which 
I isolated I reared in a series of dishes larvæ’ (about half of 
20. and R. Hertwig. Ueber den Befruchtungs und Theilungs-Vorgang des. 
Thierischen Eies unter dem Einfluss ausserer Agentien Jena 1887. 
` 3In a more extended account of my experiments I will describe in detail the meth- - 
ods which must be used in order to be certain that we are dealing with pieces with- = 
out nuclei, and on the other hand the best method te. give the more delicate pieces 
favorable conditions of development. Here I may remark that in order to obtain 
positive results, a sufficient size of the fragment is ‘necessary, also that they have @ 
nearly spherical form and sufficient amount of water for their development, and that 
MONOSPHERIC FERTILIZATION TAKES PLACE, 
