MATURATION 



143 



quite apart from the chromosomes proper (Fig. 76). It is 

 important to note that no chromosomes are lost in this way; 

 the full haploid number of these bodies remains grouped at one, 

 usually the distal, side of the nucleus. During the active 

 preparation for the first maturation or heterotype division 

 the nuclear membrane disappears, liberating the dissolved 



FIG. 76. Maturation in the egg of the Nemertean, Cerebratulus. After Coe. 

 C, D, X 375, others X 250. A. Primary oocyte. Part of the chromatin has 

 been condensed into chromosomes, only five of which are shown (the number 

 present is sixteen). The remainder of the chromatin is thrown out into the cyto- 

 plasm. The centrosomes, each with a small aster, are diverging, and the nuclear 

 membrane is commencing to disappear. B. First polar spindle fully formed and 

 rotated into radial position. Chromosomes in equatorial plate. C. First 

 oocyte division; anaphase. D. First polar body nearly separated. E. First 

 polar body completely cut off; second polar spindle formed and rotating into 

 radial position. Spermatozoon within the egg. F. Second polar body com- 

 pletely separated. Egg pronucleus forming, surrounded by large aster. Sperm 

 pronucleus, also with a large aster, enlarged and approaching the egg pro- 

 nucleus, c, chromosomes; o, nucleolus, vacuolated and commencing to disap- 

 pear; s, spermatozoon just within the egg;z\ germinal vesicle; vc, contents (extra 

 chromosomal) of germinal vesicle. /, //, first and second polar bodies; d\ sperm 

 pronucleus; Q , egg pronucleus. 



chromatin, or the extra-chromosomal masses, which then 

 disappear gradually into the cytoplasm while the small chromo- 

 somes pass into the division figure. Sometimes this loss of 

 chromatin is effected by a shrinkage of the chromosomes them- 



