PRELIMINARY GENERAL SKETCH 



195 



cleavage before the first cleavage takes place. The minute centro- 

 somes of the second cleavage are therefore the direct descendants of 

 the sperm-centrosome ; and there is good reason to believe that the 

 continuity is not broken in later stages. The facts are nearly similar 





-"i*- 



X 



•• • 







y 





• ••' • • -^ 





H 



-^ I 



% . . . . • ' 'X 



.• • • . • . . / 



7 



Fig. 99. — Fertilization in an annelid (armed Gephyrean), Thalasscma. [Grifp'IN.] 



A. Second polar body forming ; sperm-nucleus and centrosome below. B. Approach of the 

 egg-nucleus and sperm-nucleus, the latter accompanied by the sperm-amphiaster. C. Union of 

 the nuclei. D. Later stage of last. E. Prophase of cleavage-spindle. F. Anaphase of the same ; 

 centrosome divided. G. H. I. Successive stages in the nuclear reconstitution and formation of 

 the daughter-amphiasters for the second cleavage. J. Two-cell stage. 



in the trout, in Chcctopterns, and in Pliysa. ♦ In Ascaris division of 

 the centrosome first occurs at a somewhat later period (Figs. 90, 176). 

 If now the centrosomes were indeed permanent cell-organs, we 

 should thus reach the following result : During cleavage the cytoplasm 

 of the blast omeres is derived from that of the egg, the centrosouies from 



