430 T. H. MORGAN. 



embryo from the egg, it becomes much more difficult to 

 explain on the metamere assumption these variations due 

 to regeneration. 



A number of leeches were examined to see whether the 

 annuli that make up the broad metameres ever showed false 

 unions like those found in the earthworm's metameres. 

 Such modifications are not rare. 



In fig. 86, A B, the line between the second and third annuli 

 of the ninth metamere failed to extend above over to the left 

 side of the body. Again, in this same metamere we find that 

 the line between the 5th and 6th annuli turns forward to join 

 the line between the 4th and 5th, so that the last aunulus of 

 this metamere forms a compound annulus with the first 

 annulus of the next metamere, producing a short spiral form. 



A more complicated spiral is shown in fig. 87, a, b. The 

 16th and 17th metameres are involved. The spiral com- 

 mencing in the 16th metamere runs over into the next meta- 

 mere, as the reconstruction b shows. The annuli are also 

 imperfectly joined at the sides, as shown in b. This last 

 result is due to a failure of the annuli to unite perfectly with 

 one another. 



A third spiral-form, involving a double compound annulus, 

 is shown in fig. 88, a, b. A half-compound annulus above 

 and on the right side forms a spiral with a half-double com- 

 pound annulus above and on the left side. The first half- 

 compound ring is the 3rd. 



We see that the annuli of leeches show many of the same 

 sorts of false unions that are present in the metameres of the 

 earthworm, but in the leech this represents only a superficial 

 alteration. 



The modifications in the leech would correspond to those 

 modifications occasionally fo\md in the earthworm, where the 

 septa are normal but the surface markings abnormal in their 

 arrangement. 



It will be noticed that in the figures given no new annuli 

 are introduced into the metameres, and only the method of 

 union of the annuli varies. This result may only be due to the 



