236 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 
layer of the unfertilized egg contains a substance which is 
needed for development but which is not available until the 
surface layer is cytolyzed or destroyed otherwise. A further 
discussion of these possibilities with our present knowledge of 
the chemistry of the egg is futile. 
2. We have seen that the spermatozoon induces develop- 
ment by two different agencies, one of which has a membrane- 
forming effect, while the other must act somewhat like the 
hypertonic solution in our method of artificial parthenogenesis. 
We stated that the hypertonic solution has merely a corrective 
effect since the membrane formation sets the whole machinery 
of cell division into action; Boveri suggested that the spermato- 
zoon carries in its middle piece the centrosome, the real organ 
for cell division, into the egg. 
The idea that the centrosome is the middle piece of the 
spermatozoon and that the carrying of this middle piece is the 
main function of the spermatozoon in inducing development 
does not agree with the observations. F. Lillie! points out that 
the middle piece is probably not carried into the egg at all, and 
he proves that in Nereis any piece of the spermatozoon is able 
to give rise to centrosome and aster formation. These forma- 
tions arise in the egg cytoplasm which is in contact with the 
sperm fragment. The centrosome and aster formations are 
physicochemical effects induced through the influence of the 
sperm fragment. Such effects are also induced by the method 
of artificial parthenogenesis. 
Morgan? found that supernumerary astrospheres may arise 
if fertilized eggs are put into hypertonic sea-water, but the writer 
is of the opinion that this happens only if the eggs remain 
too long in the hypertonic solution. Yet it was natural to 
consider the possibility that the second factor which the sperma- 
tozoon must supply for development might be the centrosome; 
1F. R. Lillie, ‘‘Studies in Fertilization,” III and IV, Jour. Exper. Zool., XII, 
413, 1912. 
2 Morgan, Archiv f. Entwicklungsmechanik, VIII, 448, 1899. 
