BIOLOGICAL LECTURES. 



FIG. 14. Unio. Small portion of the 

 upper pole. Egg-nucleus reconstitut- 

 ing; sphere-substance forming. 



the second polar globule. At this time (Fig. 13) the sixteen 

 chromosomes that are to remain in the egg are closely packed 

 together near the animal pole, and are almost in contact with 

 the inner sphere of the aster. The inner sphere is derived from 

 the centrosome of the first maturation-spindle (see No. 8) and 



has the following structure : it is a 

 spherical body with a perfectly defi- 

 nite wall, containing a non-staining 

 substance in which is excentrically 

 placed a single centrosome united 

 to the wall by a few strands. The 

 cytoplasm in the immediate vicinity 

 is arranged in the form of an aster, 

 the fibres of which are centrally 

 attached to the inner sphere ; the 

 first row of microsomes on the radi- 

 ations of the aster bound a fairly 

 definite outer sphere. The sperm-nucleus occupies the same 

 position as before, and has undergone no change of structure ; 

 in Fig. 13 it is represented lying to the left. During the final 

 stages of formation of the second polar globule, the inner 

 sphere begins to enlarge, and the interior is occupied by a 

 reticulum, in the nodes of which are a number of centrosome- 

 like bodies. After the second polar globule is fully formed, 

 the sphere enlarges with great rapidity (Fig. 14), and, as its 

 boundary spreads out, it becomes less and less definite, until 

 its substance merges with the general cytoplasm. During its 

 enlargement, the interior is occupied by a vesicular substance, 

 at the nodes of which are deeply staining granules, in no wise 

 distinguishable from microsomes. The substance of the inner 

 sphere is now, in fact, part of the general cytoplasm. Yolk- 

 granules are entirely absent in it (Fig. 15), and this enables one 

 to follow its subsequent fate for a considerable period of time. 

 We shall call this substance, provisionally, sphere-substance , 

 following Conklin (4); it is important to remember that it is 

 derived entirely from the inner sphere of the second maturation- 

 spindle. Either this is the case, or else the apparent enlarge- 

 ment of the inner sphere at the close of the maturation is due 



