in] OTHER DEFINITIONS 69 



Then comes Experiment and confirms our con- 

 clusions of observation. The egg when it develops 

 outside the body of its parent (the rule with most 

 of the lower animals) is at the mercy of the experi- 

 menter. After it has divided into two halves, these 

 two blastomeres (as the cells produced by the sub- 

 division of the egg are called) can be separated 

 either mechanically or by chemical means. In the 

 majority of animals where this is possible, the half- 

 blastomere, that identical mass of substance which 

 without man's intervention would have formed half 

 the body of the adult, develops, owing to the mere 

 accident of separation from its sister, into a whole 

 body. Even with such a highly organized creature 

 as the newt this has been accomplished. 



The experiment may be carried still further. 

 A whole jelly-fish (Liriope) may grow up from a 

 quarter-blastomere, and in sea-urchins a single one 

 of the first 8, 16, or even of the first 32 blastomeres 



two pairs, the resemblance between the members of which is still 

 more close than that between the four taken together. This taken 

 together with the fact that the members of the pairs are always 

 adjacent seems to show that the fertilized egg divided into two 

 halves, A and B, which did not remain united. Then A divided into 

 al and a2, B into 61 and 62, and these again parted company. 

 These four cells gave rise to four separate embryos, al and a2 

 forming one pair, II and 62 the other. Thus one pair is descended 

 from A, the other from B, and the closer resemblance of the members 

 of a pair is explained by closer blood-relationship. 



