310 



DE. P. MANSON ON THE DEVELOPMENT 



and the insect may survive this operation for two or three days. 

 In such the last stage of the metamorphosis can be studied : four 

 to six days seem necessary for its completion. Out of hundreds 

 of mosquitos watched, I have been successful in finding Filarice in 

 this last stage in four instances only. In one of these there was 

 quite a number of embryos in regular gradation, from the passive 

 chrysalis up to the mature and very active embryo, so that there 

 can be no doubt of the relationship of the latter to the former, 

 though their appearances differ so much. Owing to the small 

 number of specimens I have examined, I am not quite certain 

 about the details of this stage of the metamorphosis. As far as 

 I can make out, the body gradually elongates from the hundredth 

 to the fortieth or thirtieth of an inch, and when mature it measures 

 fully a fifteenth of an inch in length by the five hundredth of an 

 inch in breadth. 



When at the above stage large cells occupy the interior ; but 

 as development advances these become reduced in size, and accu- 

 mulate round the dark line I have already mentioned as running 

 from the mouth to the caudal extremity. In this way an alimentary 

 tube is fashioned, and the peculiar and characteristic valve-like 

 terminatiom of the oesophagus in the intestine, seen in the Filarice, 

 is developed. The mouth may now be seen open and funnel- 

 shaped, and the tail is reduced to a mere stump. Movements, 

 first of a swaying-to-and-fro character, but afterwards brisker, 

 now begin. The body gradually elongates and becomes perhaps 

 slightly thinner ; all cellular appearance vanishes, and, owing to 

 the increasing transparency of the tissues, the details can no 

 longer be made out. A vessel of some sort is seen in the centre 

 running nearly the whole length of the body, and opening close 

 to one extremity ; this extremity is slightly tapered, and is 

 crowned with three, or perhaps four, papillae ; but whether this is 

 the head or tail, and whether the vessel opening near it is the 

 alimentary canal or vagina, I cannot say ; the other extremity is 

 also slightly tapered, but has no papillae. There can be no doubt 

 which is mouth and which tail, but the intermediate steps I have 

 failed to trace satisfactorily. There is a stage between these two 

 in which the mouth is closed, and the oesophagus can be seen { 

 running from it. If the body is compressed, that tube can be > 

 forced through the skin and distinctly seen ; but about that time 

 the tissues become so transparent that their exact relations can- 

 not be made out. 



I cannot say if the three or four papillae round one extremity 



