CHROMOSOMES AND ECTOPLASM 



plasmic changes which are followed by abnormal nuclear 

 behavior.^ 



The effect of the rays is limited to the superficial cyto- 

 plasm, whose altered condition brings about the remarkable 

 result that the eggs develop with seventy chromosomes 

 instead of the normal number, twenty-eight. The pro- 

 cess is as follows : Consequent to the injury of the ectoplasm, 

 the normal cytoplasmic movements are inhibited. As an 

 effect of this inhibition the first maturation-spindle formed 



/ 



<k- 





Fig. 39. — Irradiated egg of Nereis with Fig. 40. — First cleavage spin- 

 four groups of chromosomes resulting die, irradiated egg of Nereis, 

 from division of the two spindles in showing some of the seventy 

 Fig. 38. chromosomes. 



after break-down of the germinal vesicle fails to move to 

 the surface of the egg. Instead of at the periphery — to 

 which normally the spindle moves and where after separa- 

 tion of the chromosomes their outer group is extruded with 

 the first polar body — the first maturation mitosis takes 

 place at or near the centre of the egg (Fig. 37), two nuclei 

 arise and from each of these a spindle forms, each giving 

 rise to two groups of chromosomes (Fig. 38). In other 

 words, the first and second maturation mitoses take place 

 near the centre of the egg and four egg-nuclei arise (Fig. 

 39). These four nuclei with the sperm-nucleus, each con- 

 taining fourteen chromosomes, give rise in some cases to 



^ Just, ig26a, 1933b and c. 



343 



