2o8 



GYMNOSPERMS 



cry towiird the center. 'I'he gametophyte soon becomes elliptical, 

 in both longitudinal and transverse directions. In later stages it 

 takes on a pale-green color, due to the development of chlorophyll, 

 because the stony layer of the seed is rather thin and the outer 

 fleshy layer is so translucent that there is enough light for the pro- 

 duction of chlorophyll. 



The megaspore enlarges rapidly, its 

 elongated shape changing to nearly 

 spherical as the period of free nuclear 



Fig. 224. — Ginkgo biloba: free nuclei 

 in the thin layer of protoplasm lying 

 against the mc>;asporc membrane, June 

 5; X650. — .After Carothers.'o 



Fio. 225. — Ginkgo biloba: detail 

 of peripheral portion of female 

 gametophyte, July 19. The mega- 

 spore membrane is more than four 

 times as thick as on June 5. The 

 membrane about the female game- 

 tophyte, entirely independent of 

 the megaspore membrane, has been 

 formed; X650. — .\fter Caro- 



THERS.9" 



division begins. The first mitoses of the free nuclear stage are si- 

 multaneous, but at the fifth mitosis there are figures in metaphase 

 and telophase, and at the sixth, there will be some nuclei which do 

 not divide. The free nuclear period continues from the second week in 

 May to the first week in July, the number of free nuclei usually reach- 

 ing more than 256 before any walls begin to be formed (fig. 224). 



Toward the close of the free nuclear period a delicate membrane 

 forms on the outer surface of the thin layer of protoplasm. It is 

 entirely distinct from the megaspore membrane, just as the walls of 

 microspores are distinct from the wall of the microspore mother-cell. 



