308 



DE. P. MANSON ON THE DEVELOPMENT 



equivalent to about one drop of bloed from the man on whom 

 most of my observations were made, would contain from ten to 

 thirty Hsematozoa ; whereas the blood drawn by a single mosquito, 

 about as much as would fill one slide only, contained from twenty 

 to thirty as a rule, and sometimes many more. One slide, in 

 which I had the curiosity to count them, had upwards of a hun- 

 dred and twenty specimens. From this it would appear that the 

 mosquito has the faculty of selecting the embryo Mlarice ; and in 

 this strange circumstance we have an additional reason for con- 

 cluding that this insect is the natural nurse of the parasite. 



All Emhryos do not attain maturity. — By far the greater number 

 die and are disintegrated, or are expelled in the faeces undeveloped. 

 At the end of the third, fourth, or fifth day, when the stomach is 

 quite empty as far as food is concerned, and an embryo could not 

 easily be overlooked, only from two to six are found in the same 

 or slightly diff'erent stages of the metamorphosis, which I will now 

 attempt to describe. 



The Metamorphosis of the 'Embryo. — The embryo for a short 

 time after entering the stomach of the mosquito retains all the 

 appearances and habits which characterized it when in the human 

 body ; that is, it is a long snake-like animal, having a perfectly 

 transparent structureless body enclosed in a delicate and, for the 

 most part, closely applied tube, within which it shortens and ex- 

 tends itself, giving rise, from the collapse of the tube when the 

 body is retracted at either end, to the appearance of a lash at 

 the head and tail. In a very few hours changes commence. The 

 tube first separates from the body by an appreciable interval, 

 giving the appearance of a distinct double outline, and the body 

 itself becomes covered with a delicate but distinct and closely set 

 transverse striation. Oral movements are now very evident, not 

 that they did not exist before, but because the slight increase of 

 shading from the striation renders them more apparent. The 

 indication of a viscus seen in some specimens vanishes at this 

 stage. Presently the tube or sheath is either digested by the 

 gastric juices of the mosquito, or it is cast ofi" as a snake does its 

 skin, and the animal swims about naked, and without any trace 

 of a head- or tail-lash. The striation becomes very marked ; but 

 gradually as the blood thickens, and the movements of the em- 

 bryo become in consequence less vigorous, these markings com- 

 pletely disappear, giving place to a peculiar spotted appearance. 

 Each spot is dark or luminous, according to the focusing of the 



