280 



EVOLUTION AND ANIMAL LIFE 



FIG. 161. A, Lateral view of pluteus larva of 

 Echinus; B, lateral view of pluteus larva of 

 Sphiprechinus; C, hybrid pluteus of the female 

 Sphcerechinus and male Echinus. (After Boveri.) 



separated blastomeres of the two, four, eight, and even sixteen- 



cell stages of developing hydro-medusa eggs. Loeb was able 



to effect the bursting of 



n / 1\ & ^ ^ the membrane of sea- 



urchin eggs and the con- 

 sequent partial escape 

 or protrusion of parts of 

 the egg plasm forming 

 so - called extra - ovates. 

 Each of these extra- 

 ovates began develop- 

 ment as a distinct bias- 

 tula, the remainder of 

 the egg forming another 

 blast ula (Fig. 163). 

 Thus we see that experimental work has, so far, not afforded 



a positive answer to the general query proposed by the pre- 



formation versus epigen- 



esis problem. But at 



the same time it is 



obvious that the results 



of the experimental 



method are of extraor- 

 dinary interest and of 



brilliant promise. What 



seems to be revealed so 



far, is that the animal 



egg is certainly not 



rigidly preformed; that 



there is no absolute 



predetermination of the 



fate in development of 



each part of the egg 



stuff. But that nor- 

 mally in most eggs a 



given part of the egg 



does have a prospective 



definitive fate, so that 



one-half of the egg may 



be looked on as corresponding to one particular half of the 



future organism. However, the actual potentiality of any part 



FIG. 162. Cleavage of Echinus eggs in water free 

 from calcium. Note that the cleavage cells tend 

 to separate entirely. (After Herbst.) 



