ii8 Papers from the Marine Biological Laboratory at Tortugas. 



Since the suppression of skeleton, arms, and archentera, etc., do not 

 occur in control larvae, it seems reasonable to suppose that the more rapidly 

 developing larva influences the other in such a manner that the mesenchyme 

 cells of the latter are prevented from completing the skeleton, and the 

 degree of suppression seems to be related to the differential rate of de- 

 velopment, the greater the difference the greater the suppression of the 

 parts of the slower developing larva. 



The second factor making for form equilibrium is the loss of parts that 

 have already been differentiated. I have shown from the study of living 

 gastrulse that the archenteron in the slower developing gastrula in a fusion 

 may be reduced in size and completely disappear. The skeletons may also 

 actively degenerate in one of the fused larvae. This degeneration is evi- 

 denced in the reduction in thickness of various rods, in the disintegration 

 of one or both ends of rods, or both of these processes. As a result of the 

 incomplete development of the skeleton and the subsequent disintegration 

 of parts that have been formed the skeleton may ultimately consist of but 

 a single or a few independent rods. And since the size of the body is so 

 dependent upon the size of the skeleton the suppressed larva may decrease 

 in size until it almost disappears, as shown in figure 15, and I believe may 

 completely disappear. 



What happens to the tissues of the degenerating larva? Are they ab- 

 sorbed as food by the dominant larva as nurse cells supply egg cells with 

 nutriment; or are they disintegrated after the manner of certain grafts 

 without any influence upon the host; or are they transferred and recon- 

 structed within the body of the dominant larva? There is unmistakable 

 evidence that two of these processes ordinarily take place, but in unequal 

 degree and affecting different structures of the organism. 



The transfer and rebuilding of the materials of one larva into the other 

 are limited to the last differentiated tissues, namely, the skeleton. It was 

 shown that when two fused larvae developed unequally the dominant pluteus 

 may prevent the completion or the development of parts of the skeleton; 

 that under these circumstances the unused mesenchyme cells may migrate 

 to other regions of the body or into the body of the dominant larva and there 

 give rise to additional skeletal material, either by the thickening or elonga- 

 tion of the rods or by the growth of supernumerary processes and rods. 

 The extent of such hypertrophied or supernumerary growths was commen- 

 surate with the suppression of skeletal parts in the other regions. 



De Haan seems to have shown that two eggs may be completely fused 

 into one somewhat larger than the control. Driesch, Goldfarb, and de 

 Haan have demonstrated that blastulae may be completely fused together 

 into a single larger blastula. The evidence is not so conclusive but seems 

 to point to a complete fusion of gastrulae and the enlargement of the gut as 

 well as the body. 



Beyond this stage in the differentiation of the tissues a complete fusion 

 of the organs of the two larvae does not take place, at least in Toxopneustes 



