424 T. H. MORGAN. 



regenerating naturally, account for the difiFerence between 

 adults and embryos? We have seen the number of worms 

 found showing regenerated (and regenerating) posterior ends 

 to be 1 in every 13 (1 to 12) ; therefore in a lot of 225 we 

 should expect to find 19 worms having new posterior ends. In 

 our lot of 225 worms we found 100 abnormal worms. If we 

 deduct from this 100 the number of those that should have 

 regenerated tails (19), we should have 81 cases remaining. 

 The proportion would then be 81 to 225, or 1 to 2-8 (approxi- 

 mately 1 to 3). We found, however, that the proportion of 

 abnormal to normal embryos in A. foetida was only 1 to 5. 



The difference that is still found is probably due to several 

 causes, and these are not in the least of a hypothetical nature. 

 Firstly, after a time a new regenerated end cannot be distin- 

 guished from the rest of the body. Secondly, a small per- 

 centage of regeneration must also take place in the anterior 

 end. Thirdly, the difficulty of seeing the abnormalities in 

 the embryos is much greater than in the adult, and defects 

 may have occasionally escaped even a careful examination. 

 Fourthly, the data is drawn from too small a number of cases 

 to make an exact agreement very probable, even if it did exist. 



In the light of these conditions I think the closeness of the 

 result is as near as could be expected. 



A number of worms were examined in which the posterior 

 end of the body was regenerating, to see if any could be found 

 in which the tip of the "tail" showed, in process of formation, 

 modifications of the typical arrangement. Several were found, 

 a few of which are drawn in PI. 43, figs. 79 — 81. 



The first of these (fig. 79, a,b,c) shows a compound metaraere 

 in process of formation from the terminal piece or telson. On 

 the side that forms the " split" the telson is proportionately 

 longer than on the other. 



In fig. 80, A, B, c, the end of the body is very much modified, 

 and a spiral is in process of formation. The reconstruction (c) 

 shows sufficiently the conditions present. 



In fig. 81, A, B, c, another modification is present. Above and 

 on one side of the telson the outline of a metamere is seen, while 



