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



A plunger jar, such as is installed in the tank-room in the Zoological 

 Department of the Imperial College of Science, is a large glass bell-jar which 

 holds when inverted about 30 litres of water. Each jar is provided with a 

 glass cover, the upper side of which is covered by black paper, and in this 

 way the water contained in the jar is shielded from the direct action of 

 sunlight, as the tank-room is lit from above. The plunger was invented by 

 Mr. E. T. Browne, and is described by him in a paper on the rearing of 

 Medusae (2). It keeps the water in constant slow movement. 



If one compares advanced larv£e reared in plunger jars with those reared 

 in Breffit jars, one is struck with the fact that the former have f^r longer 

 arms than the latter ,and in this respect are certainly more normal, for they 

 resemble the larvae that are found in the open sea. But the production of more 

 normal larvae is by no means the only advantage which accrues from the 

 use of plunger jars. A much more important one is the far greater number 

 of larvae which can be successfully reared to an advanced stage of develop- 

 ment. In one of these jars at the Imperial College, on a previous occasion, 

 no less than 200 larvae completed their metamorphosis, and in 1917, when the 

 experiment was only carried on until the larvae were six weeks old, one plunger 

 jar alone must have contained at least 1000 larvae. If one contrasts this 

 result with that which is obtained by rearing the larva in Breffit jars, the 

 advantage of the plunger jar becomes obvious. Even if it were possible — 

 which it is not — to rear to an advanced stage 50 larvae in each Breffit jar, 

 one would require 20 such jars containing in all 50 litres of water in order 

 to accommodate 1000 larvae ; but, as every experimenter knows, it is prac- 

 tically impossible to secure exactly the same conditions in any two Breffit 

 jars ; in some the Nitzschia, for some unexplained reason, fails to flourish, 

 and as a consequence few larvae survive. For this cause, whilst I was able 

 to obtain qualitative results in 1914, 1915, and 1916, quantitative results 

 were only obtained in 1917. 



The larvae of Echinus miliaris are beautifully transparent, and were in 

 many cases drawn living. Very serviceable whole mounts were obtained by 

 preservation in carefully neutralised 40-per-cent. formalin, followed by 

 absolute alcohol. Alcoholic eosin or light green was used for staining. 



In order to elucidate the relationship between the right hydrocoele and the 

 pore-canal it was necessary to cut sections. These were taken parallel to a 

 plane cutting the larval arms at right angles. Larvae of which it was desired 

 to cut sections were not preserved in formalin, as this fluid does not give 

 sufficiently good fixation of histological details. They were preserved in 

 Bouin's fluid, which gives a very good preservation, and which makes the 

 larvae far less brittle than osmic acid followed by Muller's fluid, which I 



