THE GYMXOSPERMAE : CONIFERALES AND TAXALES 685 



MEGASPORE MEM6RANE 

 POLLEN TUBES 



NUCELLUS 



INTEGUMENT 



SPONGY TISSUE 



PERIPHERAL 

 PROTOPLASM 



MEGASPORE MEMBRANE 



mtMi.: 



mm 



EMBRYO SAC ALVEOLAR WALL /- 



MEGASPORE 

 MEMBRANE 



ALVEOLUS 



>s 



/ D 



^j( 



•^■•■■-f,' 



Fig. 685. — Pimis syhestris. Development of the female prothallus. A, Longitudinal 

 section through the nucellus of the ovoile at the beginning of the second year. 

 B, Margin of the embryo sac enlarged showing peripheral nuclei embedded in 

 cytoplasm around a large central vacuole. C, As in B, showing commencement 

 of alveolation. D, Alveoli extending into central vacuole. E, Segmentation of 

 alveoli and deposition of cell walls. F, Completed alveolation and segmentation 

 progressing to form the tissue of the female prothallus. (After Sokoloica.) 



