332 
CRESSAVELL SHEARER. 
observed by placing a capsule in a drop of sea-water on a 
slide under the low power of the microscope. Through the 
transparent wall of the capsule the males can be clearly seen 
moving about and copulating with the females, and the 
actual injection of the sperm through the body-wall of the 
females witnessed. The small size of the young worms and 
their transparent condition render it even possible to follow 
the sperm as they leave the testis and pass down the penis 
and finally lodge in the body of the female ; further observa- 
tion revealed the fact that this early fertilisation invariably 
takes place, and that it is impossible to find a free-swimming 
female, either small or full grown, that has not been 
fertilized in this manner. Moreover the males seldom leave 
the egg-capsule; after the females have hatched, they 
remain about them and die shortly afterwards. The males 
never live a true, free existence, being without mouth or 
digestive system, and subsisting during the short course of 
their life on a small quantity of yolk substance in their 
interior. They are sexually mature at the time of hatching, 
and those I have kept under observation after fertilizing 
the females died in the course of a few days. 
Starting with this observation of early fertilisation, it was 
easy to follow the course of events and observe what influence 
the sperm had on the female germ-cells as soon as these were 
distinguishable. It is not for some days after the females 
have left the capsule that the rudiments of the female cells 
appear, and this varies greatly with conditions. If the 
females do not obtain food their growth remains practically 
stationary, and after many weeks they will frequently be 
found in much the same condition as they were at the time 
of hatching. If supplied with proper food, however, they 
grow rapidly, and in four or five days the first rudiments of 
the germ-cells are distinguishable under the gut at the point 
of junction of stomach and intestine. At the time of hatching 
this triangular region is filled with a mass of clear structure- 
less tissue, in which the mass of sperm received from the 
male collects, as shown in fig. 25. At the time the female 
