No. 2.] THE ECHINODERM SPERMATOZOON. 26 1 



zoon, the centrosome becomes placed anterior, the mitosome 

 posterior to the nucleus [Platner]), it seems probable that 

 centrosome and mitosome are differentiations of one and the 

 same substance. This is th^ same conclusion reached by 

 Watase (46) arguing on another line. 



1 1 . The mitosome and centrosome represent the material of 

 the nuclear spindle (cytomicrosomes). The mitosome is that 

 portion which is of no further use, while the centrosome is 

 that part which enters with the nucleus, and is either the neces- 

 sary complement of the centrosome and spindle of the ovum in 

 fertilization, or else it alone constitutes the mechanism for 

 initiating cleavage. The differentiation of mitosome and 

 centrosome may be a device for overcoming the mechanical 

 difficulties of transferring a large quantity of spindle-forming 

 substance through the ^g^ membranes and denser outer-cyto- 

 plasmic portion. 



12. The mitosome takes no part in the fertilization process, 

 but together with the tail remains near the periphery of the 

 Qg^, and is rapidly broken down and absorbed ; while the 

 nucleus and centrosome of the spermatozoon push on towards 

 the nucleus of the egg. 



Marine Biological Laboratory, 



Woods Holl, Mass., 



Aug. 24, 1894. 



