376 



H. M. Vernon 



In the last column but one of the table are given the percentages 

 of fertilised ova which reached full larvai development. If Expts. 84, 

 86 and 118 be excluded, it appears that only 53^' of the ova 

 developed to the pluteus stage, or somewhat less than the normal 

 number (66.6^). The favourable efifect produeed by keeping water 

 in darkness is therefore not quite so great as one might conclude 

 from the increased size of the larvse. 



It has been shown that water kept in darkness may sometimes 

 become as pure, so far as its ammouia is concerned, as open sea- 

 water taken several kilometres from shore. Judged by the physiologi- 

 cal test however, its purity is not nearly so great. Thus in Exp. 69, 

 in which water collected 3 km. from shore was used, the larvse were 

 19.2^^ larger than tbe normal, aud in Exp. 134, with water 5 km. 

 from shore, Vl.l % larger than the normal. Tbe increase in size is 

 therefore considerably greater than with waters kept in darkness. 

 The number of ova reaching full larvai development was also much 

 larger, it being respectively 100^ and 95^^ in these two experiments. 



In addition to a chemical and physiological puritication, the 

 keeping of water in darkness leads to a considerable diminution of 

 the bacteria present. In the following table are given the numbers 

 of colonies, per cubie centimetre of water, formed after 24 and 48 hours 

 incubation. 



