situs inversus in echinoids 149 



Note by Professor E. W. MacBride on Mr. Hiroshi Oh- 

 shima's Paper on 'The Occurrence of Situs Inver- 

 sus AMONG artificially-reared EchinoidLarvae'. 



The most interesting paper by my friend and pupil Dr. Oli- 

 shima, which appears in this number of the ' Quarterly Journal 

 of Microscopical Science ', calls for some comment from me. 

 Dr. Ohshima refers to a paper published by me in the ' Proceed- 

 ings of the Eoyal Society ' in which I described a method for 

 inducing the formation of a second (right) hydrocoele in 

 Echinoid larvae by stimulating the larva at a critical period 

 of its growth by exposure to hypertonic sea-water. 



Dr. Ohshima states that an attempt which he made to repeat 

 this experiment in my laboratory in 1920 resulted in failure. 

 Nevertheless certain larvae with two hydrocoeles turned up, and 

 he gives a different explanation of the cause for their appear- 

 ance. I am convinced that the explanation which Dr. Oh- 

 shima gives is the right one to account for the phenomena 

 which he observed in 1920 ; but I wish to emphasize the fact 

 that his and my explanations agree in one most important 

 particular, viz. we both feel convinced that the right anterior 

 coelom of an Echinoid larva has the innate constitutional power 

 of developing a right hydrocoele. This power I account for 

 on the hypothesis that Echinoderms are derived from a free- 

 swimming ancestor provided with sets of tentacles on the right 

 and left sides of the body. Dr. Ohshima's explanation is 

 that it is a case of ' homoeosis ', but to use this term of 

 Dr. Bateson seems to me to be merely restating the difficulty 

 in other language without offering any explanation at all. 



The fact that when the right hydrocoele does appear it 

 appears in similar form to that exhibited by the left, and not 

 in the condition in which the original right hydrocoele must 

 have been when it was functional, is in my judgement to be 

 accounted for by the assumption that the modifications 

 which the left hydrocoele subsequently underwent have been 

 pushed backwards in development according to the principle 

 of tachygenesis till they now affect the earliest differentiated 



