998 THE AMERICAN NATURALIST. [Vor. XXXV. 
so that spermatozoa cannot enter from the exterior before egg- 
laying unless they make their way through the body wall. 
That this is not the case is shown very clearly by the time of 
their appearance, their relations to the remainder of the gonad, 
and by the fact that fertilization itself always occurs outside of 
the body. The structure and relations of the various parts of 
the gonads and the oógenesis afford a large number of inter- 
esting facts, but these need not be discussed in detail here. 
There is strong evidence that each gonad is always hermaphro- 
ditic after its original formation, and that egg cells and sperma- 
tozoa almost always develop together. : i 
I believe that cross-fertilization occurs very rarely in Sti- 
chostemma in nature, even if it ever occurs. The egg cells 
when they pass out of the body are immediately enclosed in 
the slime from the surface, and the spermatozoa with them. 
While it is, of course, possible that a few spermatozoa might 
make their way through the slime to the water and then again 
through the slime surrounding the eggs of other individuals, 
the most careful observations have never given the slightest 
evidence that this actually occurs. Furthermore, if it did 
occur, the second set of eggs would be fertilized before such 
a spermatozoón could reach them. 
The only possible conclusion is that self-fertilization is the 
rule in S. asensoriatum. 
As a rule only one egg. is formed in each gonad, but excep- 
tions are not uncommon, though more than two egg cells in a 
single gonad have never been observed. 
EGG- LAYING. 
The normal process of egg-laying has not been observed in 
Stichostemma, although numerous egg strings have been found 
in the jars in which the animals were kept. The extrusion of 
eggs from the body in consequence of irritation or injury has 
often been observed. If an individual containing fully grown 
eggs be cut into several pieces, the eggs, or many of them, will 
often be extruded after a short time, apparently as the result 
of contractions following the injury. The first step in the 
