ON THE DEVELOPMENT OF BILHARZIA HASMATOBIA. 645 
cule. They are probably superfluous detachments from the 
primitive yelk-mass; but it is just possible that they may 
afford some aid in the final breaking up of the shell. Whilst 
the embryo remains fixed, its tail is usually directed towards 
the narrower or spine-bearing pole of the egg; but in a few 
instances I have seen this position reversed. In some excel¬ 
lent figures of the ova of Bilharzia sent to me by Mr. 
Pilcher, I find the same variations of position faithfully 
depicted. Under high magnifying powers, whilst the em¬ 
bryo lies perfectly quiescent, the hundreds of points at which 
the rigid cilia of the body impinge upon the inner wall of the 
shell collectively present, when viewed from above, a regular 
series of highly refracting specks. These minute specks or 
dots, in certain arrangements of the focus, have the aspect oi 
so many rings, and suggest the notion of a finely sculptured 
surface, such as we see normally in the eggs of certain other 
parasites. The escape of the embryo, however, enables one 
to detect the purely optical character of the phenomenon, 
which is due to diffraction. 
As regards the precise mode of emerging from the shell, 
and the time occupied by the larva in freeing itself, there are 
some other points of interest. Speaking generally, it may be 
said that, the purer the new medium into which the ova are 
transferred, whether accidentally or otherwise, the more rapid 
and perfect will be their movements and escape. To give an 
example of observed facts in relation to the rapidity of deve¬ 
lopment, I may cite the following from my notebook. “On 
the 20th of August, 1870, I placed twelve eggs of Bilharzia 
under the field of the microscope. The medium in which they 
were immersed consisted of about eight parts of ordinary 
drinking water to one of urine. At the expiration of seven¬ 
teen minutes, the first-born, so to speak, made its escape. 
In the course of another minute, two more emerged. In 
twenty-six minutes the fourth, in twenty-eight minutes the 
fifth, in thirty-two the sixth, in thirty-four the seventh, 
in thirty-seven the eighth, in thirty-eighth the ninth, in 
forty the tenth, in forty-three the eleventh, and in forty- 
six minutes the twelfth, respectively made their appear¬ 
ance/’ 
Now this rapid mode of birth or emergence from the shell 
is very much more striking in the case of eggs which are placed 
in perfect pure water; for, whilst the eggs are still in the 
urine, there appears to be neither the power nor the incli¬ 
nation on the part of the embryo to escape; but, on iso¬ 
lating and placing them in more suitable conditions, their 
behaviour is even more remarkable. In a space of less 
