158 H. W. BEAMS. 



centrosome. The egg nucleus moves through the cytoplasm to a 

 position nearer the center of the egg (Fig. 9). 



At no time in the maturation division are all the chromosomes 

 in the same stage of division, a condition which renders it very 

 difficult to obtain the exact chromosomal count, but never has 

 there been observed in this material more than the haploid num- 

 ber after the extrusion of the two polar bodies. The extrusion 

 of two polar bodies is the usual type of maturation in Asterias 

 following artificial activation, and as Tharaldsen ('26) has shown, 

 the only type that will give rise to normal swimming larvas. 



Eggs that Extrude One Polar Body. In some eggs the first 

 maturation division is completed normally as above described, and 

 the first polar body is extruded. The polar body may be retained 

 under the limiting membrane of the egg in a compressed position, 

 or may be pinched off in the usual manner (Figs. 12-13). The 

 second polar spindle forms normally, but does not rotate in its 

 usual manner, nor migrate to the surface of the egg, but remains 

 submerged in aposition in the immediate proximity of its origin 

 (Fig. 10). Thus, it forms two daughter nuclei which are ap- 

 parently of equal size (Fig. n) and possess equal amounts of 

 chromatin material. The nuclei later fuse (Figs. 12-13) to form 

 an unusually large pro-nucleus. During the fusion of the two 

 nuclei an aster makes its appearance near the point of fusion and 

 sends its rays out along the sides of the nuclei (Fig. 12). It may 

 divide to form the amphiaster of the first cleavage spindle, as seen 

 in Fig. 14. A transverse section through the equatorial plate of 

 the first cleavage spindle of such eggs shows clearly more than 

 the haploid number of chromosomes (Fig. 15). In some cases 

 these nuclei may not fuse, but develop monasters that do not 

 divide. These eccentric monasters may complete the first cycle 

 without division and then become secondarily united with a 

 cytaster to form the second polar spindle. This spindle may 

 migrate to the surface of the egg and extrude the second polar 

 body. It is also common here too for the monaster pole of the 

 spindle to divide, giving rise to a tripolar spindle. These tripolar 

 figures in this material have never been observed to extrude the 

 second polar body. 



Eggs that Extrude No Polar Bodies. Some eggs have been ob- 



