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1907] CAROTHERS—GINKGO BILOBA 127 
9. The tetrad is usually linear, but sometimes bilateral, or a com- 
bination of bilateral and linear. 
10. The lowest spore is the functional cell. 
11. The spore is vacuolate from the first and the free nuclei of 
following stages are probably always parietally placed. 
12. Free nuclear division is at first simultaneous but gradually 
becomes irregular. 
13. Free nuclear division extends, approximately, from the second 
week in May until the first week in July. 
14. The cytoplasm, at first delicate, becomes eae and forms 
upon its outer surface a delicate wall. 
15. Centripetally growing walls are formed between the nuclei and 
with their outer edges fastened to the membrane developed at the 
outer surface of the plasmic sac. 
16. The sac fills with tissue by centripetal growth and division of 
cells, the inner ends being open. 
17. These open cells are usually uninucleate but sometimes 
multinucleate. 
18. Binucleate and multinucleate cells are frequent in young pro- 
thallial tissue, but later they become uninucleate, probably by fusion 
of the several nuclei or by degeneration of the superfluous ones. 
19. When the centripetally growing cells meet at the center each 
forms an independent end wall. 
20. The megaspore wall and the outer wall of the prothallium 
become much thickened. 
21. The megaspore wall is composed of an inner, thin, firm layer, 
and a very thick, outer layer made up of rods formed at right angles 
to the surface. 
a2. The nchepiaee are quite far ec i while there is still a 
large central cavity. 
23. The gametophyte develops ecsant chlorophyll, becoming 
the greenest tissue of the ovule. 
24. The spongy tissue surrounding the mother cell is tapetal in 
function, absorbing the surrounding tissue, and finally being itself 
absorbed by the growing prothallium. 
25. The spongy tissue cells become vacuolate and multinucleate. 
26. The integument is early differentiated into three distinct 
