20 bulletin: museum of comparatiye zoology. 



Callidina the first cleav.xge spindle is oblique to the long --^^f^^'^^f 

 therefore not in agreement with Hertwig's law ; but u.mediate y after 

 division is finished, a naovement of the egg contents takes place m 

 such a way that the two cells occupy the same relative position as in 

 r anZ,-such a position, therefore, as is demanded by Berthold's 

 fhet; of le'ast surfaces. It thus appears that in Callidina tlje irec- 

 tion of division itself is determined neither by the principle of Bertho d 

 nor that of Hertwig, but that the later arrangement of the cells might 

 be held to be due to the action of Berthold's principle. It is somewhat 

 curious that the exact arrangement produced in Callidina by shifting 

 should in Asplanchna result at once from the position of the spindle at 



the time of cleavage. 



No cause can be assigned, from the visible structure of the egg, for 

 tl,c inequality of the cleavage. The yolk granules are distributed un,- 

 JlTmly tl,rougl>„„t the egg, seeming no more abundant .n the large 

 than iu the small cell. 



Second Cleavage. 

 As a result of the first cleavase. the egg is now eoraposed of two un- 

 eo^al blnres, an anterior, /ff, and a posterior, CIX (Figs 6 and 6). 

 ^t t « smaller'blastomere, as previously sUted the -.er has already 

 divided and the two parts are separating ^J^^^":^:^ 

 cleava-e plane passes through the cytoplasm (Fig. i). The me alon 

 wh^h'th^y move apart is perpendicular to the axis of the first cleavage 

 Tel also at'right Tngles to a line connecting the polar ce 1 w, h 

 the ce;tre of the egg. The forming spindle .s thus P- ';;'"; 

 lateral axis of the embryo and consequently perpeudiculai to .t dorso 

 ven ral axis. The two asters take np their positions on opposite sides 

 Ith nucleus, and the axis of the resulting spindle has a direction 

 parallel to the line Joining the asters at their first -P--" O'."- ^ 

 and 6) Meanwhile the nucleus has steadily mcreased in size, up to the 

 time when it participates in the formation of "'YP'"'"'.^^,,, The 

 In the larger cell, (W\ the order of procedure is different. The 

 nucleus begins to enlarge, as in tlie smaller blastomere, but the a^t 

 does not at once divide. The nucleus and aster ^og^'^er begin to 

 „i„ate to the ri.ht. At the same time the aster comes to lie fa.ther 

 toihe right tha,;the nucleus, either because the two rotate on a com- 

 mon axis^ or because the aster, moving faster, creeps around '!« nucleus 

 toward L right side of it. Thus, whatever the method, a condition ,s 

 reached in which the large nucleus lies in the right anterior angle of the. 



