8o 



CELL-DIVISION- 



an attraction-sphere surrounding the centrosomes, and is perhaps 

 comparable to Heidenhain's " centrodesmus." 



In the second type, iUustrated in the cleavage of echinoderm, 

 annelid, molluscan, and some other eggs, a central spindle may be 

 formed, — sometimes already during the anaphases of the preceding 

 mitosis (Figs. 99, 155), — but afterward disappears, the asters moving 



Fig. 31. — The middle phases ot mitosis in the first cleavage of the Ascaris-^gg. [BOVERI.] 

 ^.Closing prophase, the equatorial plate forming. D. Metaphase; equatorial plate estab- 

 lished and tlie chromosomes split; b. the equatorial plate, viewed en face, showing the four chro- 

 mosomes. C. Early anaphase; divergence of the daughter-chromosomes (polar body at one 

 side). D. Later anaphase; /. ^. second polar body. 



(For preceding stages see Fig. go; for later stages Fig. 145.) 



to opposite poles of the nucleus. Between these two poles a new 

 spindle is then formed in the nuclear area, while astral rays grow 

 out into the cytoplasm. There is strong evidence that in this case 

 the entire spindle may arise inside the nucleus, i.e. from the sub- 

 stance of the linin-network, as occurs, for example, in the eggs of 

 echinoderms (Fig. 25, E), and in the testis-cells of arthropods. In 

 other cases, however, a part at least of the spindle is of cytoplasmic 



