ON THE DEVELOPMENT OF BILHARZIA HjEMATOBIA. 747 
affords the necessary leverage for ensuring the embryo's 
ultimate success. 
According to my observations, the larva never displays its 
proper elongated, spindle-shaped, or cylindro-conical figure, 
until some short time after its escape from the shell; and, as 
a consequence of this, its powers of locomotion are less 
marked at first than subsequently. At the time of extrusion, 
the larvae are commonly more or less hour-glass shaped (Fig. 
i); this particular figure being sometimes retained for many 
minutes or even for an hour (Fig. h). Usually the larvae have 
a tendency to acquire their normal shape immediately after 
quitting the shell; the oval, pear-shaped, and variously con¬ 
structed forms gradually merging into the characteristic cone- 
shaped animalcule (Figs, y, i). In their fully developed con¬ 
dition, they exhibit the most lively movements; and to 
witness several hundreds of them rushing about with unceas¬ 
ing activity is certainly a curious sight. The phenomenon, 
moreover, loses none of its interest from the consideration 
that only a few hours, or it may have been minutes, pre¬ 
viously, these now actively gyrating animalcules were lodged 
in ovo within the blood-vessels of their human host. From 
persons who are seriously infested, millions of these eggs of 
Bilharzia daily make their escape during the act of micturi¬ 
tion ; and, when this act is accomplished by the host out of 
doors, it is easy to perceive how readily the ova may be sub¬ 
jected to conditions eminently favorable to the further de¬ 
velopment of their contained larvae. The direct passage of 
the urine into any considerable receptacle of natural or fresh 
waters would in a few minutes ensure the hatching of all the 
eggs; and, in the absence of any such direct aid to develop¬ 
ment, the accidental occurrence of a shower of rain would, in 
all localities where this disease is endemic, readily transfer the 
ova into ditches, ponds, rivers, lakes, and ultimately perhaps 
even into the sea itself. Under ordinary circumstances, it is 
true, the eggs would comparatively rarely gain access to the 
sea. Moreover, as I shall show immediately, pure sea-water 
seems to be an unsuitable medium for them. 
The behaviour of the embryos under the action of reagents 
of various kinds is very remarkable. Thus, when, on the 5th 
of September, 1870, 1 placed some ova in brackish water, of 
the strength of two parts of fresh water to one of pure sea¬ 
water, their contents were readily developed, though the 
escaping embryos did not swim vigorously. When again I 
placed some other eggs in pure sea-water, their contained 
embryos became instantly transfixed, the vibratile cilia of the 
head at once becoming rigid and motionless. At first, I 
