MUSEUM OF COMPARATIVE ZOOLOGY. 295 



Between these zones lies the " middle piece " ; beyond them the " end 

 pieces." Each of these is homogeneous, slightly reddened, and only 

 rarely striate in osmic preparations ; but in chromic acid preparations 

 fine streaks are seen to connect the rods of one lateral zone witn those 

 of the others. During the constriction of the yolk, the nuclear band 

 lengthens, the two lateral zones continue to move apart, and lose th^iir 

 striate differentiation. In place of the rods are larger or smaller gran- 

 ules, and drops which have arisen by a confluence of granules, or it may 

 be a single dark red mass with a knobbed surface. The end of the 

 band is broadened, and its corners are drawn out into two prolongations 

 (Spitze), which appear as dark granules. After the completion of the 

 constriction, the lateral zones gradually become thicker and finally as- 

 sume the spherical form, and the middle and end pieces become shorter 

 and disappear by uniting with the rest of the nuclear mass. Thus the 

 nuclei of the daughter cells arise in the parts of the nuclear band called 

 lateral zones. 



These phenomena may in their interpretation be divided into two 

 groups, says Hertwig ; the one relating to the changes of the nucleus, 

 the other to those of the yolk. They accompany each other in such a 

 manner that each form of the nucleus corresponds to a definite method 

 of arrangement of the protoplasm, so that an intimate connection between 

 the two must be inferred. 



In considering whether the impulse to division proceeds from nucleus 

 or protoplasm, Hertwig says that it is from the former, and "therefore 

 considers the nucleus as an automatic centre in the cell equipped with 

 active forces." The lengthening of the nucleus is to be considered, like 

 its earlier amoeboid changes, as the result of active phenomena of motion 

 on the part of the nucleus, yet with this distinction, that the displace- 

 ment of particles is now only in two directions, instead of in all direc- 

 tions. The two poles of the nucleus exert a repulsive influence upon 

 each other, and determine the distribution of the remaining nuclear 

 mass. The two lateral zones arise out of the middle zone,* and migrate 



* A statement made by Priestley ('76, p. 152) in his review does not seem to reflect 

 in a very accurate manner the ideas of Hertwig as to the connection between the mid- 

 dle zone and the nuclei of the newly formed cells. The sentence in question is as 

 follows : — 



"Although Hertwig in his hardened and stained specimens does not certainly 

 speak of the derivation of the young nuclei from the first median thickened zones [zone], 

 there can hardly be a doubt that the lateral thickened zone[s] (which afterwards be- 

 came the nuclei) correspond entirely to the segments of the nuclear disk described by 

 Biitschli, Strasburger, and Beneden, and resulted from division of the former zone." If, 



