224 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



(Scott, 1944). At first the nuclei measure 7 to 10 microns in 

 diameter and have one to three nucleoli, but in later stages they 

 become 150 to 200 microns in diameter and their nucleoli present a 

 great variation in size, shape, and number. The following patterns 



Fig. 121. Endosperm formation in Zostera marina. A, l.s. ovule, showing mature 

 embryo sac. B, two-celled embryo and four-nucleate endosperm; note persisting 

 antipodal cells. C, D, more advanced stages in endosperm formation; note larger 

 size of nuclei in lower part of endosperm. (After Dahlgren, 1939.) 



were observed: (1) the nucleus has several nucleoli, all of a small 

 size (Fig. 123 A); (2) some of the nucleoli are small, and others are 

 of a medium size (Fig. 1235) ; and (3) most of the nucleoli are small, 

 but one to three are of a very large size and these may have a 

 spherical, elliptical, or highly erratic outline (Fig. 123C, D). In 

 later stages the largest nucleoli become subdivided into irregular 

 segments, as if in readiness to break up into pieces (Fig. 123#). 

 Since an increased number of nucleoli in a nucleus is generally 

 considered to be an index of chromosome duplication, it seems likely 



