CHAPTER VII 

 CYCADALES— Cow/mwe^ 



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EMBRYOGENY 



Just where fertilization is completed and embryogeny begins, is 

 indefinite; but the stage shown in fig. 148 is certainly regarded as a 

 stage in fertilization, while the stage shown 

 in fig. 149 shows the first division of the 

 nucleus of the fertilized egg and, conse- 

 quently, is a 2x mitosis. So somewhere be- 

 tween the stages shown in these two figures 

 fertilization has been completed, and the 

 sporophyte generation has started. 



Around the first mitotic figure there is a 

 fibrillar area many times as large as the fig- 

 ure itself. The fibrillae seem to be the same 

 as the spindle fibers and radiations of later 

 figures. The chromosomes, at metaphase 

 of the first mitosis, are not hard to count, 

 but it is surprising to find that the num- 

 ber is 12, the same number counted at the 

 mitosis which gives rise to the ventral canal 

 nucleus and the egg nucleus, and also at 

 various stages in the development of the 

 gametophyte. The anaphase of this mitosis 

 was not found, but anaphases of later mi- 

 toses showed 24 chromosomes, and root 

 tips showed the same number. The micro- 

 spore has 12 chromosomes, so there is no 

 doubt that in Stangeria, and probably in 

 the rest of the cycads, the x and 2x num- 

 bers are 12 and 24. Hutchinson's^^s work 

 on the first division in Abies, which will be considered in the proper 

 place, offers an explanation of the condition found in Stangeria. 



139 



Fig. 149. — Stangeria para- 

 doxa: first division of the nu- 

 cleus of the fertihzed egg. At 

 the top is the spiral band of 

 the sperm which fertilized the 

 egg, and also three sperms 

 which got through the neck 

 but failed to penetrate the 

 egg; X42. — After Chamber- 

 lain. "» 



