No. 2.] SPIRAL MODIFICATION- OF METAMERISM. 



249 



Fig. 3. 



etc.) were found anterior to the ex. vas. d., or in the proportion 

 as I to 25. It is highly improbable, I think, that in these 32 

 abnormalities (found in 799 worms minus 20 leaving 779) many 

 cases of an unusual position of the openings could have been 

 present ; for, as we have seen above, it occurs only once in 

 40 worms. This disturbing factor cannot, then, vitiate to any 

 great degree our general conclusion in the present case. 



One of the most instructive 

 cases is given in Fig. 3 a. This 

 shows a ventral view of a worm. 

 The loth metamere is split on 

 the left side (right of figure) 

 into two parts, remaining single 

 on the right side. The dorsal 

 view of the split-metamere is 

 seen in b. Now, corresponding 

 to this condition, we find that 

 the openings of the vasa defer- 

 entia have also been shifted, so 

 that now they come to lie on dif- 

 ferent metameres. If we count the metameres (half-metameres) 

 of the left side (right of figure a), we find that the openings 

 are on the 1 5th metamere of that side. Counting the half-meta- 

 mere of the opposite side (left of figure), we find again that the 

 opening is on the 15th of that side. That is to say, that the 

 vasa deferentia open on their proper half-metameres, and, there- 

 fore, on accoimt of the split-metamere, the two cannot be on the 

 same metamere, but on consecutive ones. 



This gives us, I believe, a clue by which to interpret the 

 split-metamere. We know that in the embryo the metameres 

 are laid down right and left of the middle line of the body 

 in blocks of mesodermal tissue; that under normal conditions 

 these hollow blocks come to lie exactly opposite (right and left) 

 of one another, so that the opposite pairs unite across the 

 median dorsal and ventral lines. If we conceive that the blocks 

 are slightly misplaced on one side, or that two consecutive blocks 

 of one side are smaller than two of the opposite side, we will 

 have as a necessary mechanical result of the relative position 

 of the blocks a split-metamere. Thus, in the last case, if we 

 suppose the loth and nth blocks of the left side smaller than 



