22 SEA-SHORE LIFE 



these is constricted off and cast out, leaving tlie 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 upertiiatozoon, 

 myriads of which are cast out into the water b}' the male starfishes 

 at the same time that the females are setting free their eggs. Each 

 spernialoz(i(jn is an exceedingly minute cell with a globular front 

 end, and a long lash-like extremity, the movements of which drive 

 it rapidly tlirough the water. 



Tlie 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 speniiatozoon 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 np of two 

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

 otlrer is maternal and belongs to the egg itself. 



After fertilization a wonderful jorocess 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 nncleus. 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 fnrrow 

 marks the position of the cleft wliich cut them in two. After a 

 few minutes of rest the egg suddenly divides again, each half lieing 

 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 hardljr 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 liall, 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 ciiange takes place, for a part of the Avail becomes 

 pressed inward as one might squeeze in tlie side of a hollow rub- 

 ber ball. This process is called "invagination," 



