328 WILSON AND MATHEWS. [Vol. X. 



the nucleus, one side of which it covers Hke a cap (Fig. 3, A). 

 In this stage ("aureole phase" of Fol) the "quadrille" should 

 occur, but not the least sign of centrosomes can anywhere be 

 discovered. Comparison of picro-osmic specimens at this stage 

 with those fixed by sublimate-acetic shows that the clear space 

 figured by Fol {e.g., in his Figs. 4-7) is an artefact, caused by 

 the destruction of the archoplasm-mass and the shrinkage of 

 the nucleus. 



The central mass now draws apart in the middle and divides 

 into two halves which lie at opposite poles of the cleavage- 

 nucleus (Fig. 3, B). Meanwhile the sperm-nucleus enlarges 

 still further, becomes closely pressed against the egg-nucleus 

 so as to assume a lens-shape, and its substance becomes retic- 

 ulated. The boundary between the two pronuclei then fades 

 away, and they ultimately fuse completely to form a single 

 reticular "segmentation-nucleus," in which it is impossible to 

 distinguish between the maternal and paternal chromatic sub- 

 stances (Fig. 3, C). 



3. Formation of the Cleavage-spindle. 



During the ensuing pause the astral rays become much 

 shorter and less conspicuous, the aster becoming greatly re- 

 duced in size. The red central mass, however, does not 

 diminish, and at this period its reticular character first becomes 

 clearly apparent (Fig. 3, C). 



About 30 min. after fertilization the rays again become con- 

 spicuous, and at this stage in favorable specimens the future 

 spindle-fibers may be clearly seen to penetrate into the seg- 

 mentation-nucleus at either pole, where the membrane fades 

 away. The remainder of the nuclear membrane now rapidly 

 disappears, and the peripheral spindle-fibers appear exactly in 

 its place, so that the spindle as a whole appears to be formed 

 by a direct transformation of the achromatic nuclear substance 

 (including the membrane). Meanwhile the nuclear reticulum 

 is transformed into the chromosomes, and the first cleavage- 

 spindle is formed (Fig. 3, D). 



To sum up : The central archoplasni-sphere (^^ attraction 

 sphere'') of the cleavage-amphiastcr is derived by direct ajid 



