29 



pressure within the capsular cavity (Fig. 61 c.o.) would throw 

 valuable light on this subject. My researches have been con- 

 cerned with the exact changes that occur in the sperm rather 

 than with the conditions that cause the changes. 



The second question, that Is the one concerning the in- 

 ternal conditions which determine the response of the sperm 

 to the external conditions, may now be considered. What is 

 there In the sperm that may react to a decrease of the osmotic 

 pressure of the solution which surrotmds it? An examination 

 of Figs. 68 to 82 clearly shows that it is the capsular cavity 

 that 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 remark- 

 able extensibility of the wall of the inner tubule which is 

 everted to form the wall of a structure many times larger 

 than the capsule. The central body must also be considered 

 as one of the structures taking a part in the explosion of the 

 capsule. We have therefore three changing structures, a swell- 

 ing mass, a stretching membrane, and an elOEgating body, each 

 of which take a part in determining the form of the Inversion. 

 To these must be added two structures which do not change and 

 are resistant in their nature. These are the wall of the 



