PLASMA-STRUCTURE IN EGG OF HYDRACTINIA 229 



During the growth-period the nucleus consists of a fine net with 

 a deeply staining nucleolus present (fig. 52), the whole very similar 

 to a corresponding stage of Hydractinia after the same killing 

 fluid (fig. 22). The nucleus grows enormously as is shown by 

 comparing figures 52 and 53, the latter being ready for maturation. 

 Because of the coagulation phenomena, I have been unable to 

 trace the reappearance of the chromosomes in this form. Clumps 

 of chromatin lying in the nuclear net give the first indication of 

 reappearing chromosomes (fig. 53). The nucleolus here, as in 

 Hydractinia, increases much in size throughout the growth-period, 

 becomes vacuolated, and disappears before the polar spindle is 

 formed. The origin of the spindle was not determined since the 

 first spindles seen were completely formed and, in the equatorial 

 plate state (figs. 52, 55), perpendicular to the surface. The 

 spindle is of much smaller size than the nucleus and is also without 

 asters and centrosomes. The chromosomes are not in the form 

 of tetrads and, since their origin is not known, nothing can be 

 said of their quadripartite condition (fig. 55). I have also too 

 few sections of the equatorial plate to establish definitely the hap- 

 loid number. Since 13 is the most constant number appearing, 

 it is undoubtedly near the reduced number (fig. 56) . Two stages 

 in the formation of the second polar spindle are shown in figures 57 

 and 58. A female germ-nucleus (fig. 59), characteristically 

 smaller than the germinal vesicle (fig. 53), is reconstructed after 

 the last polar division. The only cases of fertilization which I 

 have observed show the two germ-nuclei of equal size (fig. 60), 

 indicating that the sperm enters the egg early and so expands 

 before meeting the egg nucleus. I have found no spindle in 

 connection with the fusion nucleus, but the fact that the proto- 

 plasm killed at this period is not well fixed, may account for this. 

 Development was carried no farther, the question of cleavage and 

 amitosis not being studied in this form. The establishment of a 

 single cleavage-nucleus makes nuclear fragmentation and subse- 

 quent reorganization in 'nuclear nests' impossible and amitotic 

 cleavage improbable. 



