No. 2.] THE ECHINODERM SPERMATOZOON. 255 



tions : that the sperm centrosome is that portion of the mito- 

 some material which must be transmitted with the nuclear 

 material in the act of impregnation in order to initiate the 

 ontogeny of a new individual. The differentiation into mito- 

 some and centrosome must have taken place previous to the 

 division of the spermatogone. The question as to whether the 

 nuclei are or are not the sole participants in fertilization de- 

 pends to a considerable extent upon the decision of the question 

 of the nuclear or the cytoplasmic origin of the mitosome and 

 centrosome. It is clearly not sufficient to show that the mito- 

 some or centrosome is in the nucleus or in the cytoplasm in 

 any certain cell, but the point must be confirmed by reference 

 to as large a number as possible of successive cell generations. 



As to what is the fate of the mitosome in the process of fer- 

 tilization there has been a wide range of opinions, and if we 

 may judge from the great number of divergent observations, 

 its fate is quite different in the various animal groups, and 

 much careful work is still necessary on this point. Observa- 

 tions upon its fate in the case of the Echinoderms have been 

 made by Selenka (21), Pictet (18), Cuenot, and myself. Ac- 

 cording to Selenka's account, the mitosome becomes swollen, 

 and moving towards the female pronucleus, finally fuses with 

 it ; while the nucleus and the tail are absorbed. 



The view which Pictet brought forward, and which seems to 

 be confirmed by my preparations, is that the mitosome breaks 

 away from the nucleus, and that while the nucleus and the 

 centrosome proceed on towards the female pronucleus, the 

 mitosome and tail are left close to the point where the sperma- 

 tozoon entered the cytoplasm of the ^g%, and there break down 

 and are absorbed. The observations of Pictet and Cuenot ap- 

 pear to rest largely upon the fact that the mitosome was seen 

 in some cases previously to the penetration of the spermato- 

 zoon into the ^^^^ ^^ break off from the spermatozoon, and 

 that such spermatozoa, lacking the mitosome, are quite capable 

 of normal fertilization. My results are based upon sections of 

 artificially fertilized eggs of Asterias glacialis. 



At this point it may be well to add that though I have a 

 very great number of times observed carefully the mode of 



