282 



REDUCTION OF THE CHROMOSOMES 



(Fig. 141) but one polar body is formed, which removes eighty-four 

 dyads, leaving eighty-four in the ^^g. There may be an abortive 

 attempt to form a second polar spindle, but no division results, and 

 the eighty-four dyads give rise to a reticular cleavage-nucleus. From 



c 



"^ Ol 



wm^o ^ 



O OOpho^ 







Fig. 141. — First type of maturation in the parthenogenetic egg oi Arfemia. [BraUER.] 



A. The first polar spindle; the equatorial plate contains 84 tetrads. B. C. Formation of the 

 first polar body; 84 dyads remain in the egg, and these give rise to the egg-nucleus, shown in D. 

 F. Appearance of the egg-centrosome and aster. E. G. Division of the aster and formation 

 of the cleavage-figure ; the equatorial plate consists of 84 apparently single but in reality bivalent 

 chromosomes. 



this arise eighty-four thread-like chromosomes, and tJie same number 

 appears in later cleavage-stages. 



(b) It is the second and rarer mode that realizes Boveri's concep- 

 tion (Fig. 142). Both polar bodies are formed, the first removing 

 eighty-four dyads and leaving the same number in the egg. In the 

 formation of the second, the eighty-four dyads are halved to form 



