4 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 
unfertilized egg is unable to develop because it lacks the “organ” 
for cell divisiom According to Boveri, this organ is the centro- 
some, and it is introduced into the egg by the “middle piece” 
of the spermatozoon. During nuclear division there appear 
in the egg radiating figures, the astrospheres, whose physical 
nature is at present unknown. Some eggs, but by no means 
all, show at the center of a radiating system a granule, the 
centrosome, and in this Boveri sees the “organ of cell division.” 
According to him the unfertilized egg lacks this granule which is 
introduced at fertilization by the spermatozoon. 
Boveri’s view that the centrosomes or astrospheres cannot 
be formed by the unfertilized egg was disproved by Morgan, 
who showed, as a sequel to my experiments on the effects of 
hypertonic solutions upon the egg, that such treatment can 
lead to the formation of astrospheres in the unfertilized egg, 
and that these eggs can even begin to segment. 
In certain eggs, e.g. that of Chaetopterus, the spermatozoon 
has to accomplish another task aside from fertilizing the egg, 
namely, it must bring about the so-called maturation of the 
egg. This consists in the reduction of the nucleus by two 
subsequent nuclear divisions and the throwing-out of two frag- 
ments of the nucleus, the so-called polar bodies. For the 
maturation division, centrosomes and astrospheres are also 
required. Mead showed that in the egg of Chaetopterus 
the two astrospheres for the maturation division are present in 
the egg before the spermatozoon enters it, but that this matura- 
tion division cannot take place unless a spermatozoon does 
enter. On the other hand, the maturation division occurred 
without a spermatozoon entering, if some potassium was added 
to the sea-water. Hence the effect of the spermatozoon is in 
this case not due to the introduction of a centrosome into 
the egg. 
3. Investigations into the effects of ions had led the writer 
to believe that in them we possess very potent factors in life 
