GINKGOALES 203 



be the habit to refer to these three layers as "outer lleshy," "stony," 

 and "inner fleshy." 



At the base of the ovule, two vascular strands enter the inner 

 fleshy layer on opposite sides, through a gap in the stony layer, and 

 continue along the surface of the layer, ending where" the integument 

 becomes free. One of the characteristic features of the Ginkgo 

 testa is the suppression of one of the two sets of vascular strands 

 characteristic of the more primitive gymnosperms. In this case 

 the outer one, which traverses the outer fleshy layer, has been 

 suppressed. 



3. The gametophytes 



THE FEMALE GAMETOPHYTE 



The development of the female gametophyte (endosperm) has 

 also been investigated by Miss Carothers (46). As the functioning 

 megaspore enlarges, a large vacuole holds its nucleus near the micro- 

 pylar end. Since the megaspore, after its nucleus begins to divide, 

 is always vacuolate, the free nuclei are parietal from the beginning. 

 About the middle of May there are 16 to 64 free nuclei, well separated 

 in the very delicate cytoplasmic lining of the thin wall of the embryo 

 sac. Up to this stage (64-nucleate) the free nuclear divisions are 

 simultaneous, but later they become irregular, continuing until the 

 first of July, when there are over 256 nuclei. During this period the 

 ovule and the embryo sac are enlarging, and the megaspore membrane 

 is becoming thicker. 



Preliminary to wall-formation, a delicate membrane appears 

 on the outer surface of the protoplast of the embryo sac, which is 

 easily separable from the megaspore membrane, and may be called 

 the endosperm membrane. To this endosperm membrane (not 

 to the megaspore membrane, as has been supposed) the first walls 

 of the endosperm are anchored, being laid down at right angles to it, 

 and leaving the cells open toward the sac cavity (fig. 234). After 

 the first walls are formed, centripetal growth of the endosperm begins, 

 the innermost cells, open toward the center of the sac, always being 

 larger than the outer ones and usually uninucleate, although they are 

 sometimes binucleate and even trinucleate. In the tissue which has 

 reached nearly to the center of the sac binucleate cells are common, 



