326 Prof. E. W. MacBride. The Artificial Production of 



that there were two pore-canals and two madreporic pores, a right and a 

 left, but there was no relation to be found between the occurrence of two 

 pore-canals and two hydrocoeles. Most of the larvae with two pore-canals 

 possessed only one hydrocoele, and some of the larvse with two hydrocoeles 

 had only one pore-canal. 



The cause assigned by Gremmill for the production of two hydrocoeles, even 

 if correct, could only be operative in the case of the larvae of Asteroidea, for 

 at the stage of development at which two hydrocoeles have been observed in 

 the larvae, both of Ophiuroidea and of Echinoidea, no such extension of the 

 left posterior coelom normally takes place ; indeed, at no time in the normal 

 development of the larvae belonging to these two orders does this cavity 

 come into relation with, much less open into, the right anterior coelom. 



As the larvae of the common green shore urchin Echinus niiliaris had for 

 several years been successfully reared through their entire development in 

 the tank-room of the Zoological Department of the Imperial College of 

 Science, it seemed to me that this species would form a very suitable subject 

 on which to investigate the causes of the production of a double hydrocoele. 

 These investigations were begun in the summer of 1914, and carried to a 

 successful conclusion in the summer of 1917. 



Material and Methods. 



The specimens of Echinus miliaris used in these experiments were 

 procured from Plymouth in 1914, from the Essex coast in 1915 and 1916, 

 and in 1917 from Looe, in Cornwall. All four lots gave concordant results, 

 and so the conclusions at which I finally arrived have a considerable body of 

 evidence behind them. The best results were obtained from the Looe sea- 

 urchins, and I have to thank Dr. Allen, the Director of the Plymouth 

 Marine Biological Station, for Ins kindness in sending a special expedition to 

 Looe Island in order to secure these specimens. 



The urchins were sent to London by the swiftest route available, packed 

 in damp seaweed. A laboratory assistant awaited their arrival at the 

 railway terminus, so that the delay which would have been occasioned by 

 the slowness of delivery in London was entirely obviated. They were 

 opened and the ova and sperm were extracted from them, within an hour of 

 their arrival at the laboratory. The sea-water used in order to carry out the 

 fertilisation of the eggs and the rearing of the larvae was sterilised by the 

 method introduced by Dr. Allen in Plymouth. This method is as follows : — 

 The sea-water is first shaken up with animal charcoal and then allowed to 

 settle and the clear fluid is siphoned off. This fluid is in this way freed from 

 soluble toxins which might interfere with the growth of the larvae. It 



