264 HETEROGENETIC ORIGIN 



(i) The transformation of the entire contents of a Hydatina egg into 

 a great Otostoma. 



Having witnessed, on very many occasions, the stages of this 

 remarkable transformation of the contents of a Rotifer's egg into 

 a CiUated Infusorium, I am desii^ous of acquainting the Royal 

 Society with the simple procedure needful to enable zoologists 

 to study for themselves the series of changes leading to a result 

 which many of them may be disposed to deem incredible. 



All that is necessary is to procure a good stock of these large 

 Rotifers by placing some surface mud, having a coating of Euglense, 

 from a ditch in which Hydatinas are known to exist, into a glass 

 bowl, and to pour water very gently thereon to a depth of about 

 4 inches. In the course of two or three days (with a temperature 

 of i6° or 17° C), if the Hydatinse are abundant, a good crop of 

 their large eggs will be seen at the surface of the fluid, where it is 

 in contact with the glass. The difficulty is to find suitable sites in 

 which the Hydatinas are really abundant. An excellent source 

 from which I formerly obtained supplies has since been destroyed. 



By the aid of a scalpel passed along their track for a short 

 distance, groups of 20 to 30 eggs may be taken up at one time, 

 and gently pressed off the edge of the blade into a small, white 

 stone pot full of water. Some of such small masses of eggs (mixed, 

 perhaps, with a few Euglense) will float, and others will sink. 

 After seven or eight of these masses have been gathered and 

 deposited, the cover should be placed upon the pot so as to cut 

 off from the eggs all light rays, both visible and invisible.' 



If the supply of eggs will admit of it, two other pots should be 

 similarly charged ; but if there are not enough eggs for this 

 purpose, the two other pots should be charged on successive days 

 with fresh batches of eggs. The larger the supplies of new-laid 

 Hydatina eggs the more convincing will be the result. Thus on 

 one occasion, when my supply was very abundant, gatherings of 

 eggs were made at intervals of six hours ; and among such eggs as 

 were subsequently taken from the pots I found that from 12 to 25 

 per cent, yielded Otostomata rather than Hydatince. 



When the pots have remained covered and undisturbed for 

 36 hours at a temperature of about 17° C, one of them may be 

 opened, and some of the small masses of eggs from the bottom 



' Full details as to the best means of obtaining supplies of Hydatinse, of dealing 

 with them, and subsequently obtaining the largest proportion of freshly laid eggs, 

 will be found in my " Studies in Heterogenesis," pp. 125 and 286 



