262 HBTEROGENETIC OEIGIN 



what appeared to be a small number of half developed eggs. It 

 was replaced on the egg and water mixture, and examined again 

 after twelve hours. The fly was then dead, but its abdomen was 

 greatly distended and quite opaque owing to the presence of a very 

 large number of ova. The first fifteen that were expressed were 

 well developed ; the others less so. It contained no Ciliates. 



What the particular conditions are that lead to the premature 

 discharge of immature eggs into the abdominal cavity remains 

 altogether obscure and unknown. This, however, I am disposed 

 to think is the first event ; and the second appears to be some 

 inscrutable molecular changes occurring in these immature eggs, 

 whereby they become converted into Ciliated Infusoria. So far 

 we have to do with suppositions : but the actual facts may be 

 summarised as follows : — 



(i) In female flies that do not feed, specimens are occasionally 

 found in which a number of immature eggs of varying size are 

 found loose in the abdominal cavity, mixed with a large number of 

 embryonic Ciliated Infusoria, some of which are very much larger 

 than others, together with motionless bodies otherwise similar, 

 and similarly varying in size. 



{2) All these bodies, but especially the last two varieties, are 

 notable for the very great size of their nuclei. 



(3) Some of the motionless bodies have been seen to show 

 indications of being enclosed within a delicate cyst,' while within 

 others slowly revolving Ciliates have been seen. 



(4) The Ciliates which emerge from these delicate hyaUne cysts 

 are frail embryos (evidently the first of their race), having languid 

 movements, and provided only with short abortive cilia such as are 

 indicated in Fig. 60, A and B ; while, after twenty-four hours, they 

 assume the much more developed character to be seen in C. 



(5) By no possibility could this great number of large and frail 

 embryo Ciliates obtain an entry into the body of a fly which does 

 not feed : they must therefore be formed from some of the tissue 

 elements of the fly. 



(6) The fact that they are only found in females that contain ova 

 — never in male flies — together with the facts previously mentioned, 

 as to the similarities between the immature ova and the embryo 

 Ciliates, make the conclusion irresistible that the Ciliates are in 



• Examination of the photograph from which Fig. 59, B, was taljen with a 

 pocket lens, shows this plainly above and to the right, almost opposite the black 

 spot near that region. 



