280 



EVOLUTION AND ANIMAL LIFE 



Fig. 161. — A, Lateral view of pluteus larva of 

 Echinus; B, lateral view of pluteus larva of 

 Sph,(cr echinus; 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 



/\ jf^ -^ ^^_^ ^ ^^ 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 blas- 

 tula, the remainder of 

 the egg forming another 

 blastula (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, bo 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 iix water free 

 from calcium. Note that the cleav^age cells tenc^ 

 to gep^rate entirely. (After Perbst.^ 



