278 



EVOLUTION AND ANIMAL LIFE 



"grand problems" of heredity and variation, already well 

 entered upon, bids fair to produce the most rapid and real 

 advance that has yet been made toward the goal of solving 

 some of the mystery which has so far enwrapped these funda- 

 mental phenomena of life. 



To return to our special problem of preformation or epi- 

 genesis, it must be said at the outset that the evidence touching 

 it, which has so far been derived from experiment, is distinctly 

 conflicting. For example the frog's egg (which has been a 



classic V ersuchs object in this 

 study), when treated after its 

 first cleavage so that one of its 

 two blastomeres (daughter cells 

 of the original fertilized egg cell) 

 is killed, develops half a frog, 

 which would indicate that the 

 embryo was preformed in the 

 egg cell, or at least that each 

 part of the egg cell had its fate 

 predetermined, so that the loss 

 of part of the egg would produce 

 a loss of a definite part of the 

 embryo. 



But in the hands of other 

 investigators diametrically op- 

 posed results were got. Hertwig managed to separate entirely 

 the two first cleavage cells and got from each of these half 

 eggs a complete embryo but of dwarfed size, which would 

 indicate that any part of the egg stuff is able to produce any 

 part of the embryo. Other investigators have succeeded in 

 separating blastomeres of later cleavage stages, and have 

 variously got either miniature but complete embryos from 

 these fractional egg parts, or on the other hand parts of embryos 

 representing apparently the predetermined developmental 

 fate of the various parts of the egg. To list briefly a few of 

 these cases, we may refer to the development of partial embryos 

 from separated (2-16 cell stage) blastomeres of various Cten- 

 ophora, and the similar results with the molluscs Patella, Den- 

 talium, and Ilyanassa: to the production of defective larvae by 

 the mutilated eggs of Beroe, also of ascidians and of Echinus: 

 to Driesch's distinction between ectoderm and endoderm after 



FIG. 158. Lithium larva of the sea 

 urchin, Sphccrechinus granularis: 

 A, Elongated blastula; B, evagi- 

 nated gastrula. (After Herbst.) 



