No. 2.] ORGANIZATION OF THE EGG OF UNIO. 245 



origin of the accessory aster does not correspond with the 

 usual place of disappearance of the sperm-amphiaster ; and, 

 finally, in the egg of Crepidida (Conklin, '98a) there are two 

 or three accessory asters formed at about this time, while 

 the sperm-aster is perfectly distinct in connection with the 

 sperm-head. Carnoy ('86) has also observed accessory asters 

 of three orders in the Qg% of Ascaris megalocephala during 

 maturation. 



This negative result is the only definite conclusion at which 

 I have been able to arrive. It would seem that in this stage 

 the protoplasm of the egg is readily incited to aster formation 

 {Unio, Crepidida, Ascaris), and if one cannot refuse the name 

 of centrosomes to the central granule of such asters, it would 

 certainly seem to follow that there are present in the ^^^ other 

 bodies than the sperm- or egg-centrosomes capable of similar 

 function. 



III. Growth, Migration, and Union of the Germ-Nuclei; 



Behavior of the Sphere Substance ; Origin of 



THE Cleavage Centrosomes. 



I. Obscn^ations. 



Immediately after the formation of the second polar globule 

 the egg-sphere undergoes a most extraordinary enlargement, 

 which takes place so rapidly that it has been almost impossible 

 to find stages intermediate between Figs. ^2 and 33, PI. XXVI, 

 The interior of the sphere is filled with a very wide-meshed 

 reticulum or alveolar structure, in the walls and at the nodes 

 of which are deeply staining granules. The boundary wall of 

 the sphere can be seen during the greater part of the time of 

 expansion; but ultimately (PI. XXVI, Fig. 34) it becomes 

 stretched beyond the limits of visibility, and the sphere 

 becomes directly continuous with the general cytoplasm. 

 There is thus between the egg-nucleus and the center of the 

 Qgg at this time (Fig. 34) a mass of vesicular cytoplasm, derived 

 from the inner sphere, entirely devoid of yolk-granules. As this 

 substance preserves its individuality for a considerable time 



