NUCLEI OF ANIMAL AND VEGETABLE NUCLEI. 137 



characters of nuclei which are not as yet provided with a 

 wall. From their places of origin these peripheral pro- 

 nuclei (to use Van Beneden's expression) move towards the 

 centre with slowly increasing velocity, leaving behind them 

 a clear track as of protoplasm destitute of granules (Plate 

 XI, fig. 5) . Duirng this migration, which is probably rather 

 the result of contraction or movements of the surrounding 

 protoplasm than of forces driving the bodies through a 

 resistant mass, the nucleoli exhibit a marked activity, 

 changing their relative positions in various directions and 

 with varying velocities. At the centre the pronuclei touch, 

 at first at a point, but afterwards, owing to mutual com- 

 pression, along a straight line which runs in a direction 

 across the long axis of the %^^%. The mass thus formed then 

 rotates about its centre, so as to bring the previously trans- 

 verse line of contact longitudinal (Plate XI, fig. 6), while the 

 nucleoli one by one dissolve and vanish. The double structure 

 begins to elongate in the direction of the greater axis (Plate 

 XI, fig. 7) , in the course of which elongation the dividing line 

 of contact suddenly disappears along its whole length — a 

 phenomenon which affords a strong proof of the absence of 

 any definite pronuclear membrane. This completes the 

 coalescence of the pronuclei, and the resulting rhomb is the 

 true nucleus of the primary cleavage mass. 



Cleavage proper now begins. Elongation of the nucleus 

 continues in its former direction and a spindle is produced. 

 Soon the tips of the nucleus exhibit clearer spaces, which 

 radiate out in all directions into the granular protoplasm and 

 are united one with the other by two clear lines, one on each 

 side of the main body of the liucleus (Plate XI, fig. 8). 

 In the mean time the latter has become smaller and more 

 fissure-like, and it finally disappears altogether, leaving the 

 twin-stars with an intermediate band (Plate XI, fig. 9). 



The granules of the proper cell substance now begin to 

 recede from a portion of the wall and a lateral inversion of 

 the margin occurs (Plate XI, fig. 9). This rapidly runs into 

 a furrow which extends around the circumference of the 

 ovum — an operation in which the yolk-membrane is not 

 involved. The furrow deepens inwards and finally constricts 

 the protoplasmic mass into two. 



While the furrow is still shallow two vacuoles appear in 

 the band uniting the stars, one on each side of, and close to, 

 the plane of the equator, and commence a movement towards 

 the centres of their respective halves. Meanwhile the stel- 

 late figures in each half shi-ink, and, as they do so, the newly 

 formed vacuoles increase in size. On reaching their re- 



