SITUS INVERSUS IN ECHINOIDS 119 



in 1906 (4, p. 765), These results, considered in connexion with 

 Spemann's Triton twins and double monsters and also 

 with Morrill's double monsters of the trout referred to in 

 a foregoing page (p. 106), lead us to expect that if successfully 

 reared we might get an inverse larva from the right half of 

 the egg in these Echinoderms also. Indeed, Bpemann 

 suggested this idea at the end of his work (29, p. 413). I may, 

 however, only mention that our inverse larvae were all of 

 normal size, and that there can be no doubt as to their having 

 been developed from whole unseparated eggs. Gemmill's 

 information of several cases of twin larvae of L u i d i a s a r s i (6) 

 is not uninteresting in this respect. Eggs of early cleavage 

 stages were sent from Plymouth to Glasgow, and, according 

 to him, the long-continued shaking during the transportation 

 might have caused the blastomeres to dissociate and such 

 twins resulted. His figures, especially of those ' side-by-side ' 

 doubles (PI. ii, fig. 13 ; PI. iii, figs. 19, 21), clearly show that 

 there is no perceptible difference in structure between the two 

 halves developed from partially-separated blastomeres, nor 

 is there any sign in the right half of assuming a mirror-image 

 of the left. We cannot, however, help doubting whether 

 separation really took place during the long-continued shaking. 

 Judging from the haphazard relative positions of the halves 

 and from apparent differences in age between them in some cases, 

 one may naturally suspect that the conditions observed resulted 

 from fusion of two individuals. It is desirable to learn 

 how the left side of a member will affect the right side of the 

 other in artificially-grafted larvae. Eesults of both chemical 

 (G old far b) and mechanical (Eunnstrom) grafting of the 

 eggs or embryos are unfortunately inadequate to solve the 

 present problem. 



5. Variations among Double-Hydrocoele Larvae 



and other abnormalities. 



Our attention will naturally turn to the double-hydrocoele 



larvae which appeared in cultures associated with the reversed 



larvae. To try to find if any relation exists between these 



