144 



GYMNOSPERMS 



j)lasm about each one, segmentation begins. In some cycads, like 

 Dioon and Stangcria, an evanescent segmentation throughout the 

 entire embryo takes place before the permanent walls appear (figs. 

 154-56). In early stages they are stronger at the base of the embryo 

 and weaker and weaker above (figs. 156, 157). 



Following the free nuclear period, in the type of embryogeny 

 whicli we should regard as the most primitive, the embryo becomes 



Fig. 157. — Slangeria paradoxa: permanent walls forming below; above, weaker walls 

 and then free nuclei.— -After CHAMBERLAIN."' 



cellular throughout, as in some species of Cycas,^^^ Encephalartos, 

 and Macrozamia"^ (fig. 158). In these, the cells break down in the 

 central portion, leaving two or three layers of cells at the outside. 

 In others, including those with evanescent segmentation, the upper 

 part remains in the free nuclear condition, while cell division con- 

 tinues in the basal region. 



Dijjcrcniialion into body regions. — Hundreds of cells are formed 

 before there is any diflferentiation into body regions. The first dif- 

 ferentiation to appear is an elongation of cells which are becoming 

 the suspensor. Mucilage cavities also appear at this time (figs. 159- 



