228 COMPARATIVE ANATOMY 



free in the protoplasm and swells to twice its former size. This 

 period of growth is distinguished as Stage A, and is represented 

 in the annexed diagram (fig. 49), by the space enclosed be- 

 tween the lowest horizontal lines. The two next stages, B and 

 C, are occupied by two maturation divisions of the micro- 

 nucleus. This structure undergoes remarkable changes of 

 shape: it elongates and becomes fusiform, and then doubles 

 up to form a crescent. In the fusiform stage a granular 

 division centre can be distinguished at one of its ends and 

 from this a number of chromatin threads, connected at inter- 

 vals by short cross junctions, run to the opposite end. In 

 the crescent stage the division centre has travelled to the 

 middle of the convex side of the crescent and the chromatin 

 threads have been resolved into a number of elongated 

 chromosomes. It seems probable that each chromosome 

 is formed of two lengths of chromatin thread closely united 

 side by side, for the chromosomes are observed to divide 

 longitudinally into halves. The division centre elongates to 

 form a spindle set at right angles to the length of the crescent, 

 and division of the micronucleus follows, the horns of the 

 .crescent being gradually drawn in to the equator while the 

 halves of the chromosomes travel to either pole of the spindle. 

 This first maturation division is very possibly a meiotic divi- 

 sion. As soon as it is completed the two daughter-micro- 

 nuclei in each gamete again divide, but in this case do not 

 form crescents but long fibrous spindles (fig. 48, \ the 

 chromosomes again undergoing longitudinal division into 

 halves in the course of the mitosis. As soon as this second 

 maturation division is completed all the four daughter-nuclei 

 in each gamete begin to prepare for a new division by elonga- 

 ting to form fibrous spindles, but only one, and it is always 

 the micronucleus that happens to be nearest to the cytostome, 

 passes through the further stages of mitosis and divides. The 

 three others are arrested at the spindle stage, and then gradually 

 degenerate and are absorbed, leaving not a trace behind. In 

 the division of the surviving micronucleus the chromosomes 

 split up into rows of granules lying along the course of the 

 spindle fibres, and the granules are divided into two unequal 

 groups by a transverse division. The smaller chromosome 

 group passes into the male and the larger group into the 

 female pronucleus resulting from the division. In each 



