No. I.] MYZOSTOMA GLABRUM. 311 



with centrosomes, and that only these enter into the formation 

 of the first cleavage spindle. On the other hand, Mr. Mead's 

 studies on the egg of ChcBtopteriis show that it is the male pro- 

 nucleus alone which furnishes the centrosomes. I regard Mr. 

 Mead's observations as more satisfactory than my own, for the 

 reason that in the Annelid both the centrosomes and their enor- 

 mous radiations persist and may be followed contimtously throtigh- 

 out the whole period of fecundation and even througJioiit cleav- 

 age, whereas in Myzostoma there is a stage preceding the 

 meeting of the pronuclei, when it is extremely difficult or even 

 impossible to make out the structures in question. 



In some animals, therefore, the schema proposed by Fol and 

 Conklin appears to be considerably simplified by the suppres- 

 sion of the centrosome of one of the pronuclei. If the centro- 

 somes are really permanent cell-organs whose sole and special 

 function it is to preside over division, it is perhaps quite unne- 

 cessary that the "quadrille" should be danced in every Q.gg, so 

 long as the ^g^ is sure of receiving from one or the other parent 

 the apparatus wherewith to carry on its long series of divisions. 

 Certainly the parthenogenetic Q,gg is one of "Nature's experi- 

 ments," demonstrating the possibility of development without 

 a "quadrille." Moreover, the great physiological value of an 

 organ like the centrosome is no a priori argument against its 

 one-sided origin, since botanists have shown that the chromato- 

 phores and chlorophyll-bodies of plants — organs of very great 

 physiological value to the cells in which they occur — may, 

 during fecundation, come from one or both parents. 



University of Chicago, November, 1894. 



