2 Experiments on the Development of the Liver-Jluke. 
When the egg has just been laid, the contents are formed by 
a number of spherical masses of a clear substance containing 
many granules. These spheres consist of secondary yolk, and. 
are physiologically equivalent to the white of the fowl's egg ; 
they are often slightly polygonal from mutual pressure, and 
between them are a few bright yolk-granules. Surrounded by 
the secondary yolk-spheres, and near the anterior end of the egg, 
is a heap of pale nucleated cells with indistinct outlines, which 
represent the future embryo, and have been formed by the 
cleavage of the primitive ovum in its descent through the ovi- 
duct. The development of the embryo does not proceed beyond 
this stage whilst within the bile-ducts of the sheep ; it can only 
take place out of the body of the bearer of the adult fluke, and at 
a lower temperature. Eggs kept in water in an incubator at the 
temperature of the mammalian body do not make any progress, 
whilst other eggs kept at a lower temperature complete their 
development in a few weeks. The necessary conditions for 
development are moisture and a certain moderate degree of 
warmth ; light I have found to exert no influence. Eggs 
taken from the gall-bladder and placed directly with water into 
an opaque vessel, develop as soon as similar eggs exposed to the 
light, but otherwise kept under the same conditions. A tempera- 
ture of about 23° C. to 2G° C. is most favourable, and with this 
degree of warmth the embryo is formed in about two or three 
weeks. At a lower temperature development takes place much 
more slowly, and with an average warmth of 16° C. occupies 
two or three months. During the winter no progress is made. 
All the eggs under the same conditions, however, do not pro- 
duce embryos in the same time ; a certain number are hatched 
out on every successive day for some weeks or even months, and 
at the end of this time some of the eggs may remain in the same 
condition as when just laid. No explanation can be discovered 
in the eggs themselves of the very variable time required for 
the development of the embryo, but the fact is of much practical 
importance, for eggs scattered over any damp ground will render 
it more or less dangerous for a longer period than one year. 
The body of the embryo is formed by the growth of the 
inner cellular mass at the expense of the secondary yolk-spheres 
Avhich surround it. The outlines of the yolk-spheres become 
more distinct and their contents less granular, whilst the 
granules are often aggregated at one side of the sphere ; they 
become more fluid, and their remains gradually coalesce and 
lie on one side of the egg. The body of the embryo elongates 
and comes to occupy the whole length of the egg. A papilla 
appears at the anterior end, and behind this two annular 
furrows, between which a quantity of dark-brown pigment is pro- 
