No. 2.] ORGANIZATION OF THE EGG OF UNIO. 249 



substance (see PL XXVI, Fig. 38), I no longer believe that they 

 are descendants of the egg-centrosome. Still less can they be 

 derived from the sperm-amphiaster. / believe that they are egg- 

 products of new origin for7ned under the infliience of the two 

 germ-nuclei. It seems to me probable that each micleus has the 

 power in a certain condition of maturity to enter into a reaction 

 with the cytoplas7n, which results in the formation of an aster 

 with its centrosome, and thus initiates the process of karyokijiesis. 

 For instance, it sometimes happens in the Qgg of U^iio that after 

 the first cleavage the chromosomal vesicles unite so as to form 

 two separate nuclei in each cell, instead of a single one. After 

 these two nuclei have moved apart an amphiaster is formed in 

 connection with each, and two karyokinetic spindles arise. 



However, I have found no evidence in favor of the view of 

 Carnoy et Le Brun ('97) as to Ascaris, that the cleavage centro- 

 somes are directly derived from the nucleoli of the germ-nuclei. 

 This view would be reduced to absurdity in the case of the egg 

 of Unto, where in cleavage stages each chromosomal vesicle 

 apparently forms a nucleolus. 



The further formation of the first cleavage spindle is illus- 

 trated in PI. XXVI, Figs. 39-43. The two centrosomes move to 

 opposite ends of the plane of contact of the germ-nuclei. Bach 

 subdivides several times to form a group of granules, which rep- 

 resents the rudiment of the inner sphere. The outer sphere is 

 well marked by the absence of transverse anastomoses of the 

 rays. One of the centrosome granules then forms the center of 

 the inner sphere, and the remainder arrange themselves around 

 the periphery (Figs. 39-42). The rays of the aster form by 

 continuity of neighboring alveolar walls, as is shown in the 

 series of figures, and the central spindle is formed from the 

 small amount of cytoplasm between the germ-nuclei on the one 

 hand and the two centrosomes on the other. 



Behavior of the Chromatin in the Germ-Nuclei. 



During the growth of the germ-nuclei there is an enor- 

 mous increase in the quantity of the chromatin (PI. XXVI, 

 Figs. 34-40), but a small proportion of which is used in the 



