250 FIELD. [Vol. XI. 



The probable history of the centrosomes is as follows : the 

 original centrosome appears first in the spermatogone and is of 

 intranuclear origin (I cannot say that the centrosome is not 

 present in the spermatogones while they are still close to the 

 germinal epithelium, but I have never seen it previous to the 

 disappearance of the nucleolus ; and when first seen it seems 

 to be within the nuclear membrane). In the spermatogone it 

 divides into two, which participate in the mitotic division re- 

 sulting in two spermatocytes. With each spermatocyte nucleus 

 there is left one centrosome, which is one-half of the original 

 centrosome of the spermatogone. Initiatory to the mitotic 

 division of the spermatocyte, this centrosome divides into two, 

 which come to lie at the poles of the spindle. With the com- 

 pletion of this mitosis each spermatid contains one centrosome. 

 Thus from the original centrosome of the spermatogone four 

 centrosomes have been derived, and each of these four centro- 

 somes comes to be placed at the apex of the head of a sperma- 

 tozoon, and in the fecundation process enters the ovum with 

 the nucleus of the spermatozoon and takes part in the subse- 

 quent fertilization. 



When the nuclear membrane is formed around the nucleus 

 of the spermatid the centrosome is not included, but is left out- 

 side in the cytoplasm (Figs. 34, 36, 41, 43, 45). This fact 

 either points to the probability that the centrosome is extra- 

 nuclear in the spermatogone, or else that for some reason the 

 condition in the spermatogone varies from that in the sperma- 

 tid. If the latter is true, a possible explanation for this differ- 

 ence may be found in the fact that with the spermatid the 

 series of mitotic divisions is completed, and that subsequently 

 the constituent parts of the cell will undergo an extreme modi- 

 fication. 



In the light of the work of Fol, Guignard, and Conklin on 

 the part which the centrosome takes in the process of fertiliza- 

 tion, it is not difficult to see why the centrosome should be 

 extranuclear in the ovum and in the spermatozoon. 



In view of the fact that the centrosome is certainly 

 extranuclear in the spermatid and in the spermatozoon, 

 it is only proper to question whether it is not so also in the 



