Asymmetry in the Plutei q/" Echinus miliaris. 171 



acceptable in this case. On the other hand, we see in Gemmill's twin 

 larvae of Luidia ((2), Plate 2, fig. 13 ; Plate 3, figs. 19, 21) no indication 

 whatever of a mirror-image on the part of the right side member. Thus, if 

 it be true that the long-continued shaking caused the partial dissociation of 

 blastomeres, as he interpreted the twin-formation, and in which interpre- 

 tation there is room for some doubt, the result differs entirely from 

 Eunnstrom's. 



3. The growth of a normally developing hydrocoele might have been 

 arrested from some cause, and the right anterior coelom, to compensate this 

 defective development, produced a new hydrocoele on the right side. This 

 seems to me most probable, if not exclusive, to have occurred in our case of 

 abnormality, and it needs some further consideration. 



Erom looking through the literature of the abnormal Echinoplutei and 

 from my own observations, the following facts seem to me to have some 

 important bearings on the formation of the abnormality in question : — 



(a) Obliteration of the pore-canal (five cases in Strongylocentrotus lividus 

 by Eunnstrom, (9), p. 8; (12), p. 422; (9), p. 3; (12), p. 417, Plate 13, 

 fig. 8a; (12), p. 413, Plate 13, fig. 4; (12), p. 415, Plate 13, fig. 5; (12), 

 pp. 415-16. Two cases in Echinus miliaris by the writer. Culture No. 9, 

 " treated "). This seems to happen in connection with the shifting of the 

 pore towards the mid-dorsal line from its original position on the left side. 

 The hydrocoele, thus deprived of its communication with the exterior, becomes 

 arrested in its development, and then begins to degenerate quickly. 



(V) Activation of the right anterior ccslom in its latent potentialities of 

 producing a hydrocoele, to compensate the degenerating left hydrocoele. 

 This new structure develops exactly after the manner of the normal 

 hydrocoele on the left side. Actual observations of early stages of its 

 formation are, however, unfortunately lacking. 



(c) Eestoration to the left hydrocoele of its external communication. This 

 can take place either from the formation of a new pore-canal (two cases in 

 Strongylocentrotus lividus by Eunnstrom, (9), p. 8 ; (12), p. 422, Plate 14, 

 fig. 15 ; (12), p. 416, Plate 13, figs. 7a, I), or from fusion of the left and 

 right anterior coeloms into a single axial sinus (two cases in Echinus miliaris 

 by MacBride, (6), p. 347, Plate 5, fig. 9 ; pp. 338, 348, Plate 8, fig. 18). The 

 left hydrocoele will now continue to develop and differentiate further. 



{d) Formation of an amniotic invagination on the right side. According 

 to MacBride, this is due to the stimulating action of the right hydrocoele 

 ((5), pp. 240-41 ; (6), p. 343), while Eunnstrom is of the opinion that the 

 invagination can be formed independently of the presense of a hydrocoele. 



