414 T. H. MORGAN. 



compound metamere, still the openings of the vasa deferentia 

 appear ou their 15th half-metamere. 



It is also very noticeable that in by far the larger majority 

 of cases the false unions of the metameres are on the dorsal 

 surface. 



E. Figs. 33—46. 



The results recorded in the preceding section are, as has 

 been said, fully in accord with the statement that, however 

 varied the union of the half- metameres across the median line 

 may be, still the openings of the vasa deferentia occur in most 

 cases on their normal (15th) half-metamere. 



In the present category (e), where we should hope to find 

 this same relation to hold, the matter stands otherwise. In 

 only one case, viz. in that first given, does the anticipated 

 result follow. Many of the cases are unintelligible, or 

 nearly so. 



I used in my earlier paper the first figure referred to above 

 to illustrate the explanation oflFered as to the value of the 

 compound metamere (there called the split metamere). I still 

 believe that it does this, but I was not then aware how much 

 in the minority, as far as numbers go, this modification really 

 is. I fear that by picking out this one case as an illustration 

 in my short preliminary communication I exaggerated the 

 value of the modifications as furnishing evidence of my view. 

 I was careful to state only that this was " one of the most in- 

 structive cases, .... and gives us a clue by which to interpret 

 the split metamere." This statement I still think is true, but 

 by far the weightier evidence for my conclusion is now fur- 

 nished by the cases recorded in the preceding sections rather 

 than in this one. 



PI. 41, fig. 33, A,B, shows the anterior end of a worm having 

 the 10th metamere compound (10 — yt) ; there is, therefore, 

 one more metamere on one side of the body (the left) .than on 

 the other. Correspondingly the openings of the vasa de- 

 ferentia are not on the same metamere, but on consecutive 

 ones. Each opening is on the 15th half-metamere of its side. 

 Each half-metamere has developed its normal structures, but 



