24 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 
and difficult, as the right food of the larvae must be raised in 
the form of cultures. For our experiments we shall consider 
only the rearing of the pluteus stage. The kind of chemical 
activation of the egg, called in this book “improved method,” 
causes a development in the unfertilized sea-urchin egg which in 
a great number of cases corresponds to the form of development 
of the fertilized egg just described. The first division alone 
usually shows small irregularities, but these disappear in later 
divisions. On the whole, the reader can also apply the fore- 
going descriptions to artificial parthenogenesis. 
The experiments described in this book were performed upon 
eggs such as are deposited by the female in the water. Usually 
males and females of these forms are found together in great 
numbers, and on certain days both sexes in one region simul- 
taneously pour their sexual cells into the ocean. On the days 
on which a widely spread form spawns, the sea resembles a sus- 
pension of spermatozoa. The enormous numerical superiority 
of the spermatozoa over the eggs insures the fertilization of 
each egg. The view that the spermatozoon is chemotactically 
attracted by the egg apparently does not hold for the eggs 
of animals, although from time to time statements to the con- 
trary have been published. 
