SITUS INVERSUS IN BCHlNOlDS 143 



With regard to the occurrence of the reversed Auriculariae, 

 as discovered by Miiller (19, pp. 101, 109, Taf. v, fig. 1), 

 the attempt to interpret the phenomenon by virtue of the 

 compensatory hypertypy is nearly hopeless. It is a widely- 

 accepted fact that in Holothurians the right anterior coelom 

 does not exist at any stage throughout life, whilst the hydro- 

 coele is differentiated even before the coelomic sac divides 

 into right and left halves (posterior coeloms). It is not easy 

 to imagine that the right posterior coelom could ever produce 

 a hydrocoele, when the normal hydrocoele happened to be 

 arrested in its development. If this cannot be the case we 

 must regard it as a result either of the change of polarity in the 

 egg (according to Conklin, 3) or of twin formation (of 

 Spemann's sense, 29). 



8. External Factors as Causes of Abnormalities. 



From series of his experiments MacBride (15) came to 

 the conclusion that the chief cause producing double-hydrocoele 

 larvae of Echinus was the increased salinity of the water 

 used for culture. Unfortunately, as I have pointed out in 

 a foregoing page (p. 114), the result of our experiments of this 

 year was quite different from our expectations. As shown in 

 the table the number of double-hydrocoele larvae was greater 

 in ' controls ' than in ' treated ', i. e. 2 per cent, and 0-8 per 

 cent, respectively. As the double hydrocoele and situs inversus 

 start, I believe, under the same condition, the figures of 

 reversed larvae may also be used in this connexion. The 

 occurrence of the reversed larvae was practically equal in both 

 ' controls ' and ' treated ', i.e. 10-7 per cent, and 11-2 per cent., 

 the difference being within the range of probable error. 



Let us now turn to examine whether artificial synthetic 

 sea-water had anything to do with the production of abnor- 

 malities. Culture 11 came into contact with the synthetic 

 sea-water when the larvae were four days old, Culture 1 when 

 twelve days old, and Culture 6 when fourteen days old. They 

 were examined and counted seventy-two, six, and three days 

 afterwards respectively. Though it is unsafe to draw any 



