NUCLEOLI IN PEDICELLINA AMERICANA. 



359 



As the oocyte grows, the nucleolus increases in size, and at 

 the same time usually becomes associated with one or more of 

 the chromosomes, and about this period a small vacuole makes 

 its appearance in the center of the body. It stains very slightly 

 and stands out in marked contrast to the darker outer portion 

 (Fig. 12, b). From this point onward the course in nucleolar 

 changes is fairly direct and consists in the. constant increase of 

 the area of vacuolization until at the end of the period of growth 

 only a minimal portion of the now very large nucleolus shows 

 any stain. 



This process of vacuolization may, however, take one of two 

 courses. In the first case, there arise one or more small vacuoles 

 in the immediate vicinity of the first (Fig. 12, r), and, with the 

 growth of the nucleolus, the number of these also increases (Fig. 

 12, d\ until finally a structure of honey-comb appearance is pro- 

 duced (Fig. 12, <'). The vacuoles, which fill up the entire space 

 of the nucleolus, are now fairly uniform in size. In the second 

 case the additional vacuoles fuse with the primary one, or the 

 latter may increase in size directly. In either instance, the end 

 result is the same, viz., the central area is much increased in di- 



a 



c 



FIG. 13. 



d 



ameter, and by the time the yolk makes its appearance in the 

 egg-cytoplasm only a thin outer rim remains of the deeply- 

 staining portion (Fig. 13, a, /?). Vacuoles may, however, later 

 make their appearance in this outer rim (Fig. 13, <r), showing very 

 clearly that there is no fundamental distinction between the two 

 types above described. 



In both these groups, moreover, additional nucleoli often arise 

 secondarily, and these pass through the same processes described 

 for the primary body. In some instances these may fuse, giving 

 rise to a compound nucleolus, one part of which is generally, as 

 is shown by the differences in staining reaction, a little further 



