ii6 Papers from the Marine Biological Laboratory at Tortugas. 



larva is represented only by a short, blunt arm containing a single straight rod, 

 probably a ventral arm rod, which passes the ventral body rod of the A 

 larva and ends blindly in the common body cavity. This single rod is less 

 than one-twelfth of the complete skeleton whose parts have either been 

 suppressed or disintegrated after their formation. If either or both of these 



Fig. 14. 



Fig. 15. 



processes were to continue beyond the stage shown in this figure the blunt 

 arm would disappear with its contained single bar and all trace of the B 

 larva would be gone. It would then be absolutely impossible to distinguish 

 such a larva from the non-fused larvae in the rest of the culture. 



DISCUSSION. 



In a previous publication I have shown that two or more blastulae or 

 gastrulse derived from separately fertilized eggs, when united, tended to be 

 remolded into a single gastrula somewhat larger than the controls. In this 

 paper I have shown that this process of form regulation continued through 

 the larval period, during which the tissues were completely differentiated. 

 Such fusing larva tended to be remolded into a single one of normal size, 

 but this tendency is conditioned by certain factors, of which the following 

 are the most important : 



(i) Area of the agglutinated surfaces of the two blastiilce: When the de- 

 veloping blastulae are merely agglutinated by an inconsiderable area of their 

 surfaces, each develops into a gastrula and a pluteus complete, perfect, 



