426 Lawson . — The Gametophytes , Fertilization and 
this is accompanied by an increase in the number of free nuclei. As the 
free nuclei continue to divide, delicate membranes are formed between 
them, and they now take up a position at the inner layer of cytoplasm 
exposed to the cell-sap of the large central vacuole. This condition is 
shown in Fig. 17. The membranes formed between the nuclei are very 
delicate, and all lie more or less parallel to each other. A higher 
magnification of this stage is shown in Fig. 20. It will be seen that 
the primary cells thus formed are open on the inner side, exposing the 
cytoplasm to the vacuole. These structures are the so-called ‘ Alveoli,’ 
first described by Mile. Sokolowa (’ 91 ) in Pinus , Cephalotaxus , and 
Juniperus , and since found in other Gymnosperms by Ikeno (’ 98 ), Arnoldi 
(’00), and Coker (’ 03 ). Coker’s objection to the term ‘Alveoli’ is well 
taken, but the term he substitutes, ‘ prothallial tubes,’ is almost as mis- 
leading, for these structures are not always tubular. They constitute the 
first cells of the prothallium, and although they are open on one side and 
are later multinucleate, they are nevertheless cells. To avoid confusion 
I will therefore use the term ‘ primary prothallial cells ’ to designate these 
structures. 
Fig. 20 indicates the manner in which the primary prothallial cells are 
formed. Comparing Figs. 19 and 20, it is obvious that the nuclei have 
divided and that a delicate membrane has been formed in the region of the 
equator of the spindle, midway between the daughter-nuclei. A section 
taken parallel to the inner surface of the parietal layer of cytoplasm is 
shown in Fig. 22. This is looking down into the cells, and their outline 
and the method in which the membrane is formed are easily made out from 
this view. 
The primary prothallial cells thus organized elongate towards the 
centre of the vacuole in very much the same way as Mile. Sokolowa (’ 91 ) has 
described for other Conifers ; the nuclei retaining their position at the inner 
exposed surface. By the time the prothallium has reached the stage shown 
in Fig. 18, the nuclei in the primary prothallial cells have divided repeatedly 
and very delicate membranes have formed between them. The method of 
the formation of these membranes is shown in Figs. 20 and 21. They 
evidently develop from the continuous fibrils of the spindle which extend 
between the daughter-nuclei. As numerous cross-walls are formed, all of 
the primary cells do not extend inward as far as the vacuole. But they all 
now proceed to elongate and grow towards the centre of the diminishing 
vacuole. As this growth continues the vacuole becomes smaller and 
smaller until the space it occupied is closed by the union of primary 
prothallial cells at the centre. During this period the nuclei divide freely 
so that the cells are all multinucleate. Fig. 23 represents a longitudinal 
section of the upper position of the prothallium at this time. As shown 
in the figure, the walls of the cells are extremely delicate and may not 
