Experimentally Fused LarvcB of Echinoderms, etc. 



113 



it was an hypertrophied dorso-ventral connective of A which grew beyond 

 the limits of its own body into B. The matter can be settled conclusively 

 only by continuous observation of the living fusion, which was not made in 



D.l'C. 



Fig. ioa. 



Fig. iob. 



the case of this particular fusion. The preceding and certain of the follow- 

 ing fusions make it altogether probable that a fusion of the two skeletons 

 did not take place, but that the mesenchyme cells migrated from one part 

 of the body to another, as in figures 5, 6, 8, and 9, or from one body into 

 the other, as in figures 7 and 10. On comparing figures 7 and 10, ic will 

 be observed that the supernumerary rod in both originated in the same region, 

 is much branched, extends from one larva into the other, dififering only in 

 the matter of length, and finally that they replace the suppressed parts of 

 the skeleton of the weaker larva. The close similarity in these respects is 

 significant and supports the view of a redistribution of the mesenchyme 

 cells and the subsequent outgrowth of the hypertrophied rods into the 

 second larva. 



Figure 11, which is somewhat enlarged, might be mistaken for a single 

 larva with a supernumerary arm. On closer examination it will be found 

 that there are clearly two larvae fused very completely together. One of 

 these, the dominant or A pluteus, has the characteristic four arms, perfect 



8 



