PROCESS OF FERTILIZATION IN THE CRAB 169 
an explosion when they were transferred from a solution of one 
salt to that of another with equalosmotic pressure. Freshsperma- 
tozoa do not explode when placed in 4.25 per cent NaNO;; there- 
fore the spermatozoa must have been changed by the NaCl, or 
the presence of these two salts must have had an effect that 
neither had when acting alone. To determine the factors here 
acting will require further experimentation. 
Some of the spermatozoa explode whenever they are trans- 
ferred to a slide and covered with a cover-glass. The cause of 
such explosions was not determined. Koltzoff found that mechan- 
ical pressure would cause the explosion of the spermatozoa of 
some Decapods. I failed to produce any explosion by pressing 
on the cover-glass of a preparation containing them. Koltzoff 
(06) made extensive researches to find some specific stimulus 
that would cause a certain definite explosion which he believed 
to be the normal one but failed to find one. It appears, however, 
that a careful investigation of the conditions which initiate the 
process, followed up by an analysis of the conditions which may 
increase the pressure within the capsular cavity (fig. 61, c.c.), 
would throw valuable light on this subject. My researches have 
been concerned with the exact changes which occur in the sperma- 
tozoon, rather than with the conditions that cause the changes. 
The second question, the one concerning the internal condi- 
tions which determine the response of the spermatozoon to the 
external conditions, may now be considered. What is there in 
the spermatozoon which may react to a decrease of the osmotic 
pressure of the solution which surrounds it? An examination 
of figures 68 to 82 clearly shows that it is the capsular cavity 
which increases in size. It must therefore contain a substance 
which is isotonic with sea-water and with the blood of the crab 
and which absorbs water when placed in any solution which 
is of a lower concentration. This water is doubtless taken in 
through the wall of the inner tubule, which seems to be semi- 
permeable, while the outer wall of the capsule is probably 
impervious. 
Another striking feature of the explosion is the remarkable 
extensibility of the wall of the inner tubule which is everted 
