22- SEA-SHORE LIFE 



these is constricted olT 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 along lash-like extremity, the movements of which drive 

 it rapidly through the water. 



The globular front end of a single spermatozoon penetrates the 

 egg 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 tlie final nucleus of the egg made up of two 

 half-nnclei, one of Avhich is introduced by the male, while the 

 other is maternal and belongs to the egg itself. 



After fertilization a Avonderful 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 egg 

 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, untd 

 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 a single 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 hlastula. 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 ruli- 

 Iger ball. This process is called "invagination," 



