146 HIROBHI OHSHIMA 



happens sometimes that the left dorsal pore becomes obliterated. 

 This seems to be associated with the shifting of the pore towards 

 the mid-dorsal line. The hydrocoele, thus deprived of its 

 communication with the exterior, ceases to develop and then 

 degeneration of the whole water-vascular system sets in. 



6. The right anterior coelom, on the other hand, is now evoked 

 to realize its latent potentiality of producing a hydrocoele 

 (homoeosis). The degenerating left hydrocoele gives place to 

 a newly-appearing right hydrocoele. 



7. The right hydrocoele stimulates its adjoining tissues to 

 give rise together to an echinus-rudiment. 



8. The external factor or factors which cause the obliteration 

 of the dorsal pore could not be found. This probably is con- 

 nected with the presence of too much diatom-food and other 

 micro-organisms in the culture jar. 



9. If a new dorsal pore is formed on the left side before the 

 degeneration of the left hydrocoele sets in, the developing 

 power of the latter will thereby be revived. If sufficiently fed 

 a double-hydrocoele larva will result under such a condition. 



10. If, while the left hydrocoele is arrested in its develop- 

 ment and then degenerates, the right anterior coelom fails to 

 develop a new hydrocoele presumably from want of sufficient 

 food, a larva devoid of hydrocoele will result. 



Zoological Department, 



Imperial College of Science and Technology, London. 

 December 16, 1920. 



10. Literature Cited. 



1. Allen, E. J., and E. W. Nelson (1910).—" On the Artificial Culture 



of Marine Plankton Organisms ", ' Quart. Joum. Micr. Sci.', vol. 55, 

 no. 218 ; and also " Journ. Mar. Biol. Ass.', vol. 8, no. 5. 



2. Bateson, W. (1894). — ' Materials for the Study of Variation, treated 



with Special Regard to Discontinuity in the Origin of Species ', 

 London. 



3. Conklin, E. G. (1903).— "The Cause of Inverse Symmetry", ' Anat. 



Anz.', Bd. 23, Nr. 23. 



4. Driesch, H. (1906). — " Studien ziu- Entwicklungsphysiologie der 



Bilateralitat ", ' Arch. Entw.-Mech.', Bd. 21, Heft 4. 



