138 



MORPHOLOGY OF FERTILIZATION 



the nucleus. In all four species there is some evidence that the 

 material filling the "cave" is oriented as fibers. These structures 

 of the acrosomal region, as well as all other parts of the sperma- 



Fig. 15. Electron micrographs of thin sections of sea urchin spermato- 

 zoa. (From AfzeUus, Z. Zellforsch., 1955.) Longitudinal sections, A, 

 through head of Strongijlocentrotus droebachiensis and B, through two 

 acrosomal regions of Echinocardium cordatum. 



tozoon, are completely enclosed within a cell membrane. Afzelius 

 suggests a comparison between the acrosomal particle he ob- 

 served and the particle observed by Dan ( 1952 ) in living sper- 

 matozoa of other species of sea urchin. 



