306 ON THE INTERNAL FORM [ch. 



But this rule does not always hold; for they persist in many 

 protozoa, and now and then the nucleolus remains and becomes 

 itself a chromosome, as in the spermogonia of certain insects. 



2. Meanwhile a certain deeply staining granule (here extra- 

 nuclear), known as the centrosome*, has divided into two. It is all 

 but universally visible, save in the higher plants; perhaps less stress 

 is laid on it than at one time, but Bovery called i-t the "dynamic 

 centre" of the cellf. The two resulting granules travel to 

 opposite poles Df the nucleus, and there eacii becomes surrounded 

 by a starhke figure, the aster, of which we have sptken already; 

 immediately around the centrosome is a clear space, the centro- 

 sphere. Between the two centrosomes, or the two asters, stretches 

 the spindle. It lies in the long axis, if there be one, of the cell, a 

 rule laid down nearly sixty years ago, and still remembered as 

 "Hertwig's Law" J; but the rule is as much and no more than to 

 say that the spindle sets in the direction of least resistance. Where 

 the egg is laden with food-yolk, as often happens, the latter is 

 heavier than the cytoplasm; and gravity, by orienting the egg 

 itself, thus influences, though only indirectly, the first planes of 

 segmentation §. 



3. The definite nuclear outhne is soon lost; for the chemical 

 "phase-difference" between nucleus and cytoplasm has broken 

 down, and where the nucleus was, the chromosomes now he (Figs. 

 90, 91). The lines of the spindle become visible, the chromosomes 

 arrange themselves midway between its poles, to form the equatorial 

 plate, and are spaced out evenly around the central spindle, again 

 a simple result of mutual repulsion. 



4. Each chromosome separates longitudinally into two|| : usually 

 at this stage — but it is to be noted that the spHtting may have taken 

 place as early as the spireme stage (Fig. 92). 



* The centrosome has a curious history of its own, none too well ascertained. 

 The ovum has a centrosome, and in self- fertilised eggs this is retained; but when 

 a sperm-cell enters the egg the original centrosome degenerates, and its place is 

 taken by the "middle-piece" of the spermatozoon, 



f The stages 1, 2, 5 and 6 are called by embryologista the prophase, metaphase, 

 anaphase and telophase. 



X C. Hertwig, Jenaische Ztschr. xviii, 1884. 



§ See James Gray, The effect of gravity on the eggs of Echinus, Jl. Exp. Zool. v, 

 pp. 102-11, 1927. 



II A fundamental fact, first seen by Flemming in 1880. 



