274 HETEEOGENETIO ORIGIN 



invariably met with where the contents of the eggs are being 

 transformed into Otostomata, though nothing like it, as I have said, 

 is to be found during the normal development of the Hydatina 

 egg. I have found eggs in a very similar condition in two others 

 of this group of dead Rotifers ; but since then, though I have many 

 times searched for them, I have failed to find eggs developing, either 

 normally or abnormally, within dead Hydatinas. This is rather 

 remarkable, and in great contrast with what obtains among the 

 dead Callidinas from the Luchon lichen.' 



Some explanation of this difference may possibly be found in 

 the different media from which these two kinds of Rotifers are 

 taken. The Hydatinas commonly exist in more or less foul waters, 

 and, perhaps as a consequence, after their death they are found to 

 rot rapidly and disappear, leaving in the course of two or three 

 days only a heap of granular debris, the nature of which is indicated 

 merely by the presence of the pharynx — which resists decay for a 

 longer period. With the Callidinas, however, even after the lichen 

 has been soaking in distilled water for six or seven days, the 

 medium is different. Although it may and does commonly contain 

 plenty of Bacilli, the bodies of these dead Rotifers will remain in 

 the fluid for many days without undergoing disintegration, and 

 during this period any eggs that they may contain grow and 

 develop. That eggs, which are small at the time of the death of 

 the CalUdinse, grow so as to attain their full size is evidenced by 

 the fact that though, as with Hydatinae, it is rare to see these living 

 Rotifers bearing more than one fully-developed egg at a time, it is 

 quite common, as I have shown, to find within them, when dead, 

 from two to four eggs of equal size and in different stages of 

 development. 



These facts seen to point to the conclusion that the Hydatinas 

 found by me in this one experimental pot may have come from 

 some medium purer than usual, and that they must have died 

 nearly simultaneously, from some unknown reason. They con- 

 sequently did not rot so quickly, and there was time for the eggs 

 to grow and undergo this or that stage of development. Once 

 afterwards I found some Hydatinze, which had been placed in a 

 watch-glass about thirty-six hours previously, dead and containing 

 also two or three equal-sized eggs. What had caused them to 



' See "Studies in Heterogenesis," pp. iig-124, for an account of the trans- 

 formation of the eggs of these Rotifers into a different kind of Ciliate belonging 

 to the genus Glaucoma. 



