CHAMBERLAIN— STAN GERI A PARADOX A 



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An examination of the tiny Zamia pygmaea might show a com- 

 paratively short free nuclear period, but I have not yet been able 

 to secure it. Ginkgo, with its comparatively small eggs, has only 

 7 free nuclear divisions, the walls appearing with the 8th division. 

 Selaginella and Isoctcs have a short period of free nuclear division 

 at the germination of the megaspore, but none of the living hetero- 

 sporous pteridophytes has any free nuclear division in the embry- 

 ogeny. These stages have not yet been described in an)- fossil 

 heterosporous pteridophyte, but some of the paleozoic forms had 

 megaspores much larger than those of Isoctcs or Selaginella . Doubt- 

 less, with the increase in the size of the egg, a free nuclear period 

 became established, becoming more and more extensive as the 

 egg became larger, until it reached its maximum in some of the 

 living cycads, which have eggs 5 or 6mm. in length, and which 

 may have more than 1000 free nuclei before wall formation begins. 

 Among the coniferophytes, Ginkgo has the most extended free 

 nuclear period yet known. While we believe that the Ginkgo con- 

 dition represents a culmination, it is not so clear that genera with less 

 and less extended free nuclear periods in the embryogeny represent a 

 reduction series, ending in forms like Sequoia, in which a cell wall 

 follows the first mitosis in the fertilized egg. This may seem rather 

 speculative, but we believe enough has been observed to warrant 

 our theory that the origin and development of the free nuclear 

 period has been due to a progressive increase in the size of the egg. 

 Migration of nuclei.— As soon as the first free nuclear period 

 has come to a close, many of the nuclei in the upper and middle 

 portions of the proembryo migrate to the bottom (figs. 16, 17, 18, 

 27). That nuclei actually move to the basal region is shown by 

 the structure of the cytoplasm and arrangement of the nuclei, and 

 also by the relative numbers of nuclei in the upper and lower parts 

 of the proembryo. The cytoplasm becomes vacuolated in the lower 

 portion, before any movement of the nuclei begins (fig. 15); but 

 during the movement and in succeeding stages the vacuolated 

 condition extends throughout. As the nuclei move to the base of 

 the proembryo, the cytoplasm at that point becomes denser and 

 denser, until it contrasts rather sharply with the vacuolated structure 

 above (fig. 18). 



