120 Papers from the Marine Biological Laboratory at Tortugas. 



(2) Time of fusion: The earlier in development that fusion occurred the 



more completely did the parts tend to fuse together and vice versa. 



(3) Area of common surface : The greater the surface common to both eggs 



or blastulae, the more completely and the earlier did they tend to 

 fuse, and vice versa. 



(4) Differential rate of development: When the two embryos developed 



equally fast, each was essentially perfect, though fused to the other. 



When one developed less rapidly than the other, reciprocal changes 



took place, as a result of which the larvae tended toward complete 



fusion and form equilibrium. 

 Form equilibrium is determined by at least these three factors, namely : 

 (i) Differential rate in the development of the two eggs. 



(2) An inhibition in the development of certain structures. The skele- 

 ton in the more slowly developing larva was rarely complete, lacking those 

 portions of the skeleton that were last differentiated. Such portions may 

 include one-half, three-quarters, or more of the skeleton. The parts that 

 do appear are perfectly normal. 



(3) A disintegration of certain structures. Skeletal parts of the larva 

 after their complete differentiation may break down and completely dis- 

 appear. Such decrease in the skeleton was accompanied by a decrease in 

 the corresponding parts of the body of the larva. 



Such regulative changes are nearly always limited to the more slowly 

 developing larva. They result in the complete atrophy of this larva without 

 any reciprocal or compensatory effect upon the shape, size, or parts of the 

 more rapidly developing or dominant larva, except for the skeleton. 



The ectoderm and the entoderm of one larva may fuse with that of the 

 other, and may subsequently be absorbed or be disintegrated without any 

 visible effect upon the other larva. The skeleton never fused. When it 

 was suppressed in one larva, some or all of the unused mesenchyme cells 

 migrated to other regions of the same larva, or into the adjoining larva, and 

 there gave rise to hypertrophied or supernumerary parts of the skeleton, 

 and in certain instances replaced in part the missing skeleton of the sup- 

 pressed larva. 



