﻿i9i6] CHAMBERLAIN— STANGERIA PARADOXA 365 



free nuclei in the upper half of the proembryo were often smaller 

 and more numerous than those in the lower half. The explanation 

 was soon found, for the nuclei in the upper half may divide simul- 

 taneously, while those in the lower half remain in the resting con- 

 dition (fig. 14). This would double the number of nuclei in the 

 upper part, and naturally the nuclei would be smaller. A counting 

 of nuclei in several complete series of sections indicated that such 

 a division had taken place. When the lower nuclei divide, the 

 upper ones divide also, even if they are already much more numer- 

 ous (fig. 13). There is one important exception to this statement 

 which will be considered later. 



While such nuclear conditions are frequent, they are not uni- 

 versal, for two series showed approximately 512 nuclei of uniform size 

 and uniformly distributed throughout the egg, thus indicating that 

 there had been 9 simultaneous divisions. With the 9th or sometimes 

 the 10th nuclear division the free nuclear period comes to a close. 



At any time after the 7th division there may be an evanescent 

 segmentation of the protoplasm, especially in the upper part of the 

 proembryo (fig. 15). We have already described such a condition 

 in Dioon edule (12), and while we have not yet published the details, 

 we may state that in Macrosamia, Encephalartos, and Cycas the 

 segmentation is much more complete than in Stangeria or Dioon, 

 and is more permanent, since cellulose walls are formed. 



As already stated, the haploid and diploid numbers of chromo- 

 somes are 12 and 24. The number 24 is fairly stable in the lower 

 part of the proembryo; but in the upper part there are frequent 

 and extreme variations, the numbers ranging from the theoretical 

 24 down to a single chromosome. If many of the figures should 

 show just 12 chromosomes, the number could be explained by the 

 continued division of an extra sperm, the first division of which 

 has already been noted; but no such cases were observed. The 

 cause of the irregularity was not determined. 



Polarity. — A striking feature of the free nuclear period is the 

 frequency of a definite polarity. During the earlier divisions, the 

 nuclei may be fairly evenly distributed, but still in two recogniz- 

 able groups (rig. 12). Very often, however, the grouping is con- 

 spicuous, with a considerable space separating the upper and lower 



