248 LILLIE. [Vol. XVII. 



have met, it is again in evidence in determining subsequent 

 movements to the place of origin of the cleavage spindle, and 

 later in determining the ultimate position of the latter. I am 

 convinced, from its subsequent behavior, that the penetration 

 path of the sperm-head is conditioned by dynamic relations with 

 the egg-substance dependent on the orientation of the latter. 

 " The paths of the germ-nuclei are determined by at least two 

 different factors, one of which is an attraction or other dynami- 

 cal relation between the nuclei and the cytoplasm, the other 

 an attraction between the nuclei. The former determines the 

 entrance path of the sperm-nucleus, while both factors probably 

 operate in the determination of the copulation path, along which 

 it travels to meet the egg-nucleus. The real nature of neither 

 factor is known" (Wilson, '96, pp. 151 and 152). 



Origin of the Cleavage Centrosotnes. 



When the two germ-nuclei come together there is no indica- 

 tion of an aster in any part of the egg. But, after they have 

 been in contact for a short time, two centrosomes arise quite 

 near together, one in contact with the egg-nucleus and the 

 other with the sperm-nucleus, but both close to the plane of 

 contact of the two (PI. XXVI, Fig. 38. See description of 

 this figure). I feel quite sure that the two centrosomes do not 

 arise from the division of one immediately preceding, but are 

 formed independently ; for there is no trace of a central spin- 

 dle between them, and at this stage not more than two rows of 

 cytoplasm alveoli are involved in the rudimentary asters. All 

 of my efforts to find in earlier stages any indication of origin from 

 a single centrosome, either in closer approximation of the two, 

 or in union by a central spindle, have been in vain, in spite of 

 a superabundance of good material covering the critical period. 



I was under the impression at the time of my first paper on 

 this subject {Science, March, 1897) that the cleavage centro- 

 somes arose in the sphere substance, and thought it possible, 

 therefore, that they might be traced back to the egg-centro- 

 some of the maturation divisions. But, although the centro- 

 somes do arise very close to or on the margin of the sphere 



