Asymmetry in the Plutei q/" Echinus miliaris. 175 



MacBride ((6), p. 326). One feature is, liowever, certainly common to the 

 double-hydrocoele larvae of these three classes of Echinoderms, namely, the 

 temporary arrest of the growth of the left hydrocoele in an early stage, from 

 some cause or other. And this occurs more frequently under artificial 

 conditions than in nature. 



What then is the external factor which causes the arrest of growth of the 

 left hydrocoele in the Echinoid larvaj ? It seems to me that the obliteration 

 of the pore-canal is a direct cause of the arrest, and that obliteration of the 

 canal is associated with the presence of too much diatom food and other micro- 

 organisms in the culture jar, which arouses either mechanical or physiological 

 disturbance in the normal development of the larva. The phenomenon in 

 which the latent potentiality of producing a hydrocoele of the right anterior 

 coelom is provoked has been ingeniously compared by MacBride to artificial 

 parthenogenesis ((6), p. 341). He thus regards the hypertonic sea water as 

 one of the chief factors producing the double hydrocoele. This most interest- 

 ing hypothesis is not as yet fully proved, and it is desirable that it should be 

 set free from criticisms through further test. 



Note hy Prof. E. W. MacBride, F.E.S. 



My friend Dr. Ohshima and I agree in our fundamental explanation of 

 the development of two hydrocoeles in the larva of Echinus miliaris. "We 

 both attribute this phenomenon to a latent power in the right anterior 

 coelom developing a hydrocoele, a power which is normally inhibited by the 

 development of the functional hydrocoele from the left anterior coelom. 

 This latent power of the right side, which is also manifested in the larvse 

 of Asteroidea and Ophiuroidea, I believe to be an indication that the 

 common bilateral ancestor of the Echinodermata had, corresponding to the 

 hydrocoele, a paired organ equally developed on both sides of the body, 

 and that, whilst the organ on the left side became further developed until 

 it grew to be the water-vascular system and its appendages, the organ on 

 the right side dwindled and disappeared. 



Dr. Ohshima does not accept this explanation, but prefers to explain the 

 development of a right hydrocoele as " homoeosis." I think this is really 

 to substitute a word for an explanation. What is meant by homoeosis ? 

 The word was coined by Bateson (1894) to signify the phenomenon which 

 appears in the meristic repetition of seriate organs, viz., that a segment 

 sometimes acquires the characters of a member of the series preceding or 

 succeeding it. But this phenomenon is now more fully understood; it 

 depends on the formation of tertiary organs — as we may call them — under 

 the stress of formative stimuli. Thus the proto-vertebrse of the developing 



