228 LILLIE. [Vol. XVII. 



the general theory of the permanence of the centrosome as 

 the special organ of cell division, or the center of organic 

 radii. Brief abstracts of the results were published in the 

 form of two short papers, read before the American Society of 

 Morphologists in the winter meetings of 1897 and 1898, sum- 

 maries of which appeared in Science in March, 1897 and 1898. 

 The second paper was also published in full in the Zoological 

 Bulletin, Vol. I. Part of the subject was described for the first 

 time in my paper on "Adaptation in Cleavage," in the Bio- 

 logical Lectures for 1898 (Lillie, '97-'99). 



Since this work was begun the entire centrosome question 

 has entered on a new phase of development, and the subject 

 now appears to me to be in such a condition that the great 

 need is for careful observations rather than for any attempt 

 to found new theories or defend old ones ; any such attempt 

 in the present state of our knowledge would be premature. 

 It is in this spirit that I publish the following detailed account 

 of my observations. The work, moreover, has had for me a 

 greater embryological than purely cytological interest ; hence 

 I lay the greatest weight on the evidence as to the organiza- 

 tion of the Q^^. The conclusions concerning this subject link 

 themselves directly to my earlier observations ('95) on the cell- 

 lineage of Unio, and are a further elaboration of the principles 

 laid down in that paper. 



The ^%^ of Unio is most definitely oriented. The orienta- 

 tion is not due to the arrangement of the deutoplasmic mate- 

 rials, for the yolk-granules have no definite abiding-place in 

 this Q.%g, but are driven here and there by cytoplasmic move- 

 ments. Yet of all the many hundreds and even thousands of 

 eggs examined, not one was found in which the polar globules 

 were not formed opposite to the micropyle, which thus occu- 

 pies the center of the vegetative pole. It is here, of course, 

 that the spermatozoon enters. 



The eggs are fertilized in the suprabranchial chamber by 

 spermatozoa that enter with the inhalent current of water. 

 Apparently, contact with the water may act as stimulus for the 

 first breaking of the germinal vesicle ; because, so far as my 

 observations go, the spindle is not formed in the ovary, and I 



