184 



GENERAL EMBRYOLOGY 



G 



FIG. 95. Fertilization in 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 cytoplasm. 

 The centrosomes, each with a small aster, are diverging, and the nuclear mem- 

 brane is commencing to disappear. B. First polar spindle fully formed and ro- 

 tated 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 completely separated. 

 Egg pronucleus forming, surrounded by large aster. Sperm pronucleus, also 

 with a large aster, enlarged and approaching the egg pronucleus. G. Approach 

 of the two pronuclei. Egg aster reduced, sperm aster greatly enlarged and 

 centrosome divided. H. Fusion of pronuclei; divergence of the sperm 

 centrosomes. /. First cleavage figure in early anaphase. Chromosomes divided 

 and beginning to diverge; centrospheres enlarged, c, chromosomes; o, nucleolus, 

 vacuolated and commencing to disappear; s, spermatozoon just within the egg; 

 v, germinal vesicle; vc, contents (extra-chromosomal) of germinal vesicle; /, //, 

 first and second polar bodies; d\ sperm pronucleus; 9 , egg pronucleus. 



