68 - THE AMERICAN NATURALIST. [Vor. XXXIX. 
mal development of the egg, and sometimes even proving fatal. 
Abnormal development also follows where the egg-nucleus 
unites with more than one sperm-nucleus. 
Thus the entrance of a spermatozoón into an egg cannot be 
regarded as in itself the efficient stimulus to normal develop- 
ment. Moreover, it is possible by etherizing an egg, to prevent 
the union of the sperm-nucleus with the egg-nucleus. By this 
means one half of the egg, that containing the egg-nucleus, 
develops parthenogenetically while the other, containing the 
sperm-nucleus, develops merogenetically. Knowing all this, one 
must realize that the simple fact that a spermatozoön has entered 
an egg is relatively unimportant, though it is of course a neces- 
sary step in fertilization, rendering possible the union of the. two 
pronuclei. 
I consider it therefore perfectly correct to hold to the old 
definition of fertilization and to call an egg fertilized only when 
the union of the nuclei is accomplished. This definition does not 
take into account the behavior of the centrosomes, and rightly, 
since these constitute only the stimulus, and the controlling 
apparatus of division and are not as important as the chromo- 
somes, 7. e, whether the division is accomplished through the 
influence of the egg-centrosome, or that of the sperm-centrosome, 
does not change thé outcome of the union of the pronuclei. 
Applying the same standard to a definition of parthenogenesis, 
we should expect to say that the latter represents development 
without fertilization ; yet we shall see that such a definition is 
too broad and that in order to avoid misunderstandings it is 
necessary to restrict it in certain points, 
Development without fertilization takes place normally in 
many animals of different groups. Leaving out of considera- 
tion reproduction by the division of the whole animal, we find 
these three kinds of propagation: parthenogenesis, paedogenesis, 
and budding. Under budding we understand development 
through successive, regular mitotic cell-divisions, proceeding 
from one or several cells. Parthenogenesis is always devel- 
opment of an unfertilized egg and is introduced by a matura- 
tion process, while the nature of paedogenesis is not yet known. 
The facts that peedogenesis occurs in only a few animals and 

