380 DR. P. MANSON ON THE METAMORPHOSIS OF 



41, 42 are from the same insect. I have seen and sketched specimens resembling figs. 3, 

 8, and 12 in the same field of the microscope, and lying across each other. 



In the little caudal appendage that characterizes the Filaria from the second to the 

 fifth stage we have a strong argument for believing we are concerned with the same 

 species during all these stages. It ceases, however, to be distinctive towards the end of 

 the fifth and during the sixth stage; but here a curious circumstance comes to our 

 assistance in establishing identity. The thick cellular body of the fourth and commencing 

 fifth stages is motionless ; but as the anterior part fines down and stretches out, it at the 

 same time acquires movement. A difference also obtains between these two parts a& 

 regards their capacity for being stained with gentian violet. The posterior and cellular 

 part takes the stain with difficulty, or not at all ; whereas the anterior fibrous and active 

 part takes the stain rapidly and deeply. Now we sometimes find a Filaria with a long 

 stretch of the fore part of the body in thickness and general appearance like that of fig. 31, 

 and in very active movement, dragging after it a limp and seemingly paralysed posterior and 

 cellular part, rupturing readily on immersion in water, and showing all the characteristics 

 of the cellular stage. Further, on applying the stain to such a specimen, the active fore 

 part is deeply affected, whereas the limp hind part is but slightly tinged * . Such a 

 specimen bridges over the step from figs. 23 to 33. Catching specimens such as fig. 27 

 and fig. 28 in the act, so to speak, of stretching, distinctly establishes that the Filaria oi 

 the sixth stage is but an advanced form of the Filaria of the fourth, and, by a similar j 

 process of reasoning, of the first, and of the Filaria in the human blood. 



I can imagine that some may found an objection to the theory of the action of the 

 mosquito on the fact that, of the hundreds of Filaria; entering a certain mosquito, only 

 a dozen or so will attain the more advanced stages of development ; and that, of those 

 that do thus advance in development, most perish in consequence of the death of the 

 insect before the Filaria is equipped for independent life. It is true that the great 

 majority cease to develop at a very early stage ; others, after advancing a little, seem to 

 die and wither. Fig. 36 is from such a specimen. Perhaps only certain tissues or 

 organs of the mosquito favour complete development, and in their blind wandering few 

 of the Filarice reach these. But these facts are no argument against the theory that the 

 mosquito is the intermediary host of the Filaria : indeed, on the contrary, they are in 

 entire consonance with what Ave know of the life-histories of other parasites. Of the 

 millions that leave the parent, but one or two ever reach maturity. The journey from 

 the uterus to the spot where maturity is attained is beset with dangers, the measure of 

 these dangers being the prodigious numbers that start on the journey. 



* Strange to say, the fully developed Filaria, as that represented in figs. 32, 33, does not take the stain well. It 

 seems to me that the age of the integument has something to do with the facility with which the stain is ahsorhed. 

 Specimens, as at tigs. 22, 23, 25, do not, stain well, I believe the reason being that the integument in these is old and, 

 presumably, dense. Hut the anterior half of fig. 27 would stain well: it is the growing half, and, I think, has just 

 undergone ecdysis, and the new skin is easily permeated by the dye. Its posterior half, however, will not stain at 

 all, or but very slightly and unevenly ; the old skin is not yet cast, and is dense and impervious. The new skin, 

 after a time, as it ages, also becomes dense, so that by the time the animal has grown to the size of the original of 

 fig. 33, it is impervious, and will hardly allow the passage of the gentian violet. 



