410 T. H. MORGAN. 



In fig. 16 a similar spiral involves the llth^ 12th, and 13th 

 metameres. Vasa deferentia normal. 



In fig. 17 a spiral is present that involves the 12th, 13th, 

 and 14th metameres. The lower end of the figure (posterior 

 end of the spiral) turns to one side and ends in a point. The 

 openings of the vasa deferentia were not apparent on the 

 surface, but dissection showed the ovaries were in the 13th 

 metamere, that is within the spiral. Presumably, then, the 

 vasa deferentia were also in their proper metamere. 



In fig. 18 a longer spiral involves the 6th, 7th, 8th, and 

 9th metameres. The spiral belongs to category x (PI. 40, 

 fig. x). This form does not introduce any extra half-meta- 

 meres, and we find the vasa deferentia normal in position. 



In fig. 19 a similar (reversed) spiral is found. It includes 

 the 11th, 12th, 13th, and 14th metameres. The vasa defe- 

 rentia open normally. 



Conclusion. — In these six cases where a spiral is intro- 

 duced in front of the 15th metamere the position of the 

 openings of the vasa deferentia is not aflfected. No better 

 demonstration is needed to show that the spiral is due simply 

 to a rearrangement of the segments involving their union 

 across the middle line. It will also be noticed that in all of 

 these cases the false union is made on the dorsal side of the 

 worm. 



2nd case. — In fig. 20, a, b, c, are drawn the upper and 

 lower surfaces of a worm, and (in c) a reconstruction of the 

 spiral. The 2nd, 3rd, and 4th metameres are involved, and 

 an extra half-metamere (3rd) is introduced upon the left side 

 of the body. The modification belongs to category vii 

 (PI. 40, fig. VII ). We find corresponding to this intercalation 

 that the openings of the vasa deferentia are aff'ected, aud 

 appear on different but consecutive segments. Each, however, 

 opens on its own 15th half-metamere. In other words, the 

 15th half-metameres still develop their proper reproductive 

 openings, and in consequence of an interpolated half-metamere 

 in the spiral anteriorly the two openings do not fall into the 

 same metamere. 



