no Papers from the Marine Biological Laboratory at Tortugas. 



It is well known that in the development of the skeleton the mesenchyme 

 cells differentiate two triradiate spicules, one on each side of the body. 

 The bars of the triradiate spicules develop unequally. The ventral body 

 rod, the ventral arm rod, and the dorso-ventral connective are formed from 

 the outgrowth of the original three bars. Subsequently the dorsal body 

 rod and the dorsal arm rod are differentiated, and finally the aboral branches 

 of the ventral body rods. In the two examples of fused larvae just described, 

 it was found that, when skeletal parts are suppressed, such suppression is 

 limited to the parts last to be differentiated, namely one or both dorsal rods 

 and the aboral branches of the ventral body rods. That this suppression 

 was permanent can readily be demonstrated. It will be recalled that all 

 parts of a pluteus are fully formed within 72 hours, yet these fused plutei, 

 after a period of 4 to 7 days, had not differentiated their missing rods. 



Fig. 7a. 



Fig. 7b. 



The last fusion does not offer such clear evidence of compensatory 

 growth as the preceeding fusion; it is probably limited to the extremely 

 elongated dorso-ventral connectives of the B pluteus. 



In the last two instances the fused plutei were equal or nearly equal, 

 both in size and in the stage of development. In a large number of instances, 

 however, the two plutei are unequal in both these respects and in varying 

 degrees of inequality. For example, figures 7A and 7B are two views of the 

 same pair of fused plutei, of which figure 7A is somewhat enlarged. In 

 these figures the A pluteus is clearly larger and more completely differenti- 

 ated than its mate; the four arms, the shape of the body, and the archen- 

 teron are completely differentiated and normal. The B pluteus appears 

 to have but one abbreviated arm and a diminutive archenteron. 



The skeletons show very clearly the unequal development of the two 

 plutei. The A pluteus contains all the skeletal parts of the normal pluteus, 



