THE STAJiFISn 247 



only a small part of the body is divided off to form the next 

 generation. In this case the living thing thus separated from 

 the parent is not only very small but, apparently, very simple. 

 It undergoes a series of changes by which it becomes more 

 and more complex until finally the adult form is achieved. 

 The study which traces the changes that the egg passes through 

 in its development into the adult is known as embryology. As 

 an example of the embryological history of an organism we 

 may consider the development of the starfish egg. 



Development. — Starfishes have the sexes distinct; and 

 the sex can sometimes be told by the color. In the common 

 Eastern species ' the female has a bluish tint, while the males 

 are of a reddish brown color. Both the eggs and sperm are 

 extruded into the water through small openings on the aboral 

 surface at the interspace between two adjacent arms. The 

 egg is hardly visible to the naked eye and is enclosed in a 

 delicate membrane through which one of the sperms penetrates 

 into the egg. This is called fertilization. The entrance of 

 the sperm seems to start the development of the egg. First, 

 the egg undergoes cleavage ; that is to sajr, the single sphere 

 breaks up into two, four, eight, sixteen spheres, and so on 

 (Fig. 236). Finally a hollow ball, made up of these spheres 

 or cells, is formed, called the " blastula." Next the wall of 

 this hollow sphere is pushed in at one side, forming a sort of 

 cup with two walls. This is the so-called " gastrula." The 

 cavity of this cup is the digestive cavity. At first it is a sac 

 with only one opening, but later a second opening, the true 

 mouth, breaks through and the former opening persists as 

 the anus. Pairs of arms, edged with cilia, now bud out on 

 each side of the body. At this stage the organism is alike 



' Asterias vulgaris. 



