THE BEGINNING OF DEVELOPMENT 39 



Still active, since it is sometimes found that a gene 

 has mutated, or changed into one of its other forms, 

 in a body-cell of a late embryo, and it is then still 

 capable of affecting the few daughter-cells which 

 subsequently arise from it. 



Spemann's experiment in which he made artificial 

 twins raises the whole question of whether half an 

 c^gg will always develop as well as a whole egg, and 

 if so why an egg usually develops into only one adult 

 and not two. But it will be more convenient to 

 postpone discussing this problem (see Chapter vi) 

 till we have dealt with the next period of develop- 

 ment, the gastrulation period, in which the main 

 outline of the embryo is formed. When the cleavages 

 finish the tgg is a blastula, a hollow ball of cells. 

 The blastula-cavity appears quite early in eggs with 

 a total cleavage, since the daughter-cells remain 

 more or less spherical and leave a space in the 

 middle, just as tennis balls would if they were packed 

 together. This space grows until it is much larger 

 than the individual cells, which continually decrease 

 in size as the cleavages proceed. In large yolky eggs 

 with a discoidal cleavage the hollowness is not so 

 obvious, but the disc of cells lifts slightly away from 

 the mass of yolk and leaves a narrow space which 

 corresponds to the blastula-cavity (Fig. 6). The 

 next chapter describes how this hollow ball of cells 

 is converted into a three-layered gastrula. 



