^77 
««prual fjpnemtionn occur before the sexual stage is again reached, 
! hesitate to pronounce a hasty conclusion. At any rate, the 
prr^we of fully develo|)e(i niirncuHa In latcral-spined eggs is a point 
which still requires to \>e cleared up biologically. For the question 
ai to the s{>ecific nature of the latcral-spined ovum the point is of no 
twsequence; for its combined presence in the same individual with 
the termmal-spincd egg is evidence enough that only individual 
oioditHMis can be responsible for its formation. 
I will now try to show that the strange and striking difference 
f^errd hy the clinical and pathological pictures of Bilharr.iosis in 
nnous places is not incapable of explanation if we consider the 
presumptive life history of the parasite, in connection with 
the habits of the host and the conditions 
of the country. In order to make this clear I must start 
Irom the beginning. 
The miracidium (often inappropriately called ‘ embryo ’) contains 
tn Its abdominal cavity the so-called ‘ germinal cells,’ the significance 
and ultimate fate of which arc well known from their comparative 
it«^- in various other Trematodes. The existence of these cells m 
the Biiliarzw niiracidiuin is absolute evidence that the 
nurscidmm cannot develop directly into an 
idult worm, hut must pass through the stage of the sporo- 
cyst ■ which, in its turn, produces, cither (and probably) at once, or 
by one or more intermediate generations, the definite w'orms. All 
»ttanpt8 made by former authors to discover an intermediary host 
m which this development is gone through, have failed, and so have 
tny own efforts. 1 have examined hundreds of specimens of all 
the molluscs common in the Nile valley, without finding any sporpeyst 
•brh might have liern brought into relation with the Bilharzia 
•wmi I have placed quantities of free swimming miracidia in 
with the same molluscs, without obtaining an infection. It 
" easy to infect molluscs with miracidia of species which actually 
dcwfcp in them I will not enter into details, but only say that the 
*Whanu miracidia were never seen to take any notice of any mollusc 
• their neighbourhood, whereas others developing in a certain 
•“fc* soon begin to swarm about it. and may, under the microscope, 
be otnen'ed to enter into it The same negative results weie 
«**ained with larvae of insects, with fishes, and with plants. I am 
