22 SEA-SHORE LIFE 
these is constricted off and cast out, leaving the egg with only a 
half-nucleus. 
The egg is then mature and ready to be fertilized. This is 
accomplished by a single male germ cell, called a spermatozoon, 
myriads of which are cast out into the water by the male starfishes 
at the same time that the females are setting free their eggs. Each 
spermatozoon is an exceedingly minute cell with a globular front 
end, and a long lash-like extremity, the movements of which drive 
it rapidly through the water. 
The globular front end of a single spermatozoon penetrates the 
ego and fuses with the half-nucleus; and it is most interesting to 
observe that this front end of the spermatozoon is itself a half- 
nucleus, in appearance similar to the half-nucleus of the egg. In 
this manner then is the final nucleus of the egg made up of two 
half-nuclei, one of which is introduced by the male, while the 
other is maternal and beiongs to the egg itself. 
After fertilization a wonderful process sets in. This is called 
cleavage or cell division. First of all the nucleus divides into two 
similar nuclei, and these separate while at the same time the ege 
becomes cut into two, so that each half contains a nucleus. The 
halves of the egg do not, however, remain far apart but apply them- 
selves closely one to another, so that soon only a shallow furrow 
marks the position of the cleft which cut them in two. After a 
few minutes of rest the egg suddenly divides again, each half being 
cut into two, and this process is repeated again, and again, until 
we have a great number of little cells all gathered together into 
a ball hardly larger than the original egg. 
Soon, however, we see that the ball is not a solid mass of cells, 
for the centre becomes hollow and filled with fluid, while the cells 
range themselves round the central cavity in asingle layer. The 
creature thus becomes a hollow ball, the wall of the ball being 
composed of a single layer of cells. It is then that we first observe 
any active movement on the part of the embryo, for the outer sur- 
faces of the cells become covered with minute hair-shaped lashes 
the rapid movements of which cause it to spin through the water. 
This little, hollow, ball-like embryo is called a blastula. Soon an 
interesting change takes place, for a part of the wall becomes 
pressed inward as one might squeeze in the side of a hollow rub- 
ber ball. This process is called “invagination,” 
