250 MORGAN. [Vol. VII. 



the loth of the opposite side, the three half-metameres of these 

 blocks will form together a split-metamere. Now, if the vasa 

 deferentia still appear, each on its proper half-metamere, or 

 mesodermic block, they must, as explained above, appear at a 

 different level. Moreover, we gather from Fig. 3 that the half- 

 blocks 10 and 1 1 of the left side of the worm are each equiva- 

 lent to a simple block of the opposi^te side, as is indicated by 

 the displacement of the openings of the vasa deferentia. It 

 follows as a corollary, therefore, that the half-blocks 10 and 11 

 of this side cannot be looked upon as a single half-metamere 

 itself, divided into equivalent halves ; for, if this were true, we 

 would not expect to find the shifting of the reproductive 

 organs. I think, therefore, it is more correct and simpler to 

 fall back upon the interpretation given above, and to regard the 

 two half -blocks 10 and 11 as true half-metameres. That this is 

 the correct interpretation of the splitting is further shown in 

 the fact that the splitting always takes place in the sides, right 

 or left, and never entirely above or below. 



Moreover, we have seen that the number of abnormalities is 

 greater in regenerated parts than in the embryos, and this finds 

 its explanation as soon as it is seen that the formation of the 

 body cavities is here a much more irregular process (as I have 

 determined in regenerating metameres) than in embryonic 

 growth. Furthermore, the same explanation that accounts for 

 the split-segment is equally applicable to the spiral modifica- 

 tion. If we imagine one of the mesodermic blocks of one side 

 to be larger either above or below (but not both above and 

 below), so that above (let us say) it opens into two body cavities 

 of the opposite side, while below it opens into but one, then we 

 have produced the conditions necessary to start the spiral. 

 Each of the consecutive blocks on the same side as the larger 

 block will open below into its proper opposite block ; but above 

 (on account of the first displacement) into the one lying in 

 reality behind it. Reference to Fig. 2 a and c may make this 

 clearer ; but it is difficult to put into words what a model will 

 show at a glance. The spirals when once started do not run 

 on continuously, but end after- passing around the body several 

 times. The ending likewise finds its explanation in the in- 

 equality of the blocks of opposite sides. I have found these 

 spiral methods of metameric growth in widely separated genera 



