434 T. H. MORGAN. 



expectation. If the facts recorded in the preceding pages 

 indicate anything clearly^ it is that the union of the blocks 

 across the middle line is the natural result of their size and 

 of their position. Under normal conditions the blocks are 

 so placed that they unite pair for pair. If the order is dis- 

 turbed they unite differently. This is brought out most 

 strikingly in that case where there were 134 half-blocks on 

 one side and 118 half- blocks on the other. Here throughout 

 the body there are very few half-segments that are united with 

 the one opposite bearing the same numerical relation from 

 before backwards. 



In the light of these statements I think we ought not to 

 look to any one half-metamere as predestined to carry under 

 all conditions the openings of this or that organ. Rather 

 should we expect that those segments, which happen to get so 

 placed in the body of the worm that they correspond to the 

 normal position for a particular organ, will go ahead 

 and develop that organ. We might say that the position in 

 which an organ will develop is determined by a definite region 

 of the body irrespective of how that region has gotten into the 

 necessary position, provided this happened when the cells were 

 still undifferentiated. The question is too large to discuss 

 here, and the facts too meagre. 



There is a liability to err if the statements made above in 

 regard to the methods of union of the metameres be applied 

 to all cases. All that I contend for is that the majority of cases 

 will bear this interpretation. Sooner or later one is sure to 

 come across individuals where it is impossible to see the appli- 

 cation of the theory. Particularly have I found this true in 

 the PolychsELta. I have met such cases quite often in examin- 

 ing the regenerated anterior ends of worms where the head had 

 been artificially cut off very obliquely. Sometimes pieces are 

 left, where the new head joins the old body, that cannot be 

 interpreted as half-metameres. Whether all of these unusual 

 cases are the result of regeneration I am not prepared to say. 

 Many of them seem to be ; others, so far as I know, may have 

 come in from the egg. 



