288 Lawson . — The Gametophytes and Embryo of the 
prothallial cells (‘ alveoli 5 ) gradually encroach upon the space occupied by 
the vacuole, and as nuclear division continues numerous cross-walls are 
formed. The majority of the nuclei, however, are found on the inner 
exposed side of the primary cells, and seem to completely surround the 
diminishing vacuole in the manner shown in Fig. 15. It will be seen that 
the formation of the prothallial tissue in Libocedrns is essentially the same 
as that described by Coker (’04) for Taxodium . In Taxodium , however, 
Coker finds that cell-formation usually begins in the archegonial region 
earlier than it does in the basal region of the prothallium. This is not the 
case in Libocedrns , for the upper and basal regions develop permanent 
tissue about the same time. The central region seemed to be the last to 
develop cell-walls. In several cases traces of the diminishing central vacuole 
were observed, even after the archegonia had been organized. 
In his investigation of the gametophytes of Thuja occidentalism Land 
(’02) did not study the development of the megaspores or the formation 
of the endosperm. Coker (’04), however, has given a brief account of the 
development of the megaspores in Tlmja orientalis . My own observations 
of the megaspores of this latter species of Thija do not agree entirely with 
those of Coker, although they do in the main. Coker finds that the single 
mother-cell becomes evident among the conspicuous spongy tissue, with its 
long axis often at right angles to the longitudinal axis of the ovule. This 
single mother-cell gives rise to four spores, which are arranged in the 
form of a tetrad. They are never arranged in a straight row, but are 
packed close together in such a way that only two or three nuclei may be 
seen in one section. I was unable to confirm these observations from my 
own preparations. In the first place I find that the tapetum, especially in 
the mother-cell stage, is not very conspicuous. I also find that there are 
generally two spore-mother-cells developed. Coker, however, states that 
occasionally two, or even three, may be differentiated. Many of my 
preparations show eight megaspores arranged in a single group, and they 
resemble very closely those which I have figured and described above for 
Libocedrns. 
In regard to the development of the prothallium in Tlttya, as yet no 
observations have been recorded. I have therefore carefully followed out the 
formation of the endosperm in T . orientalis , and record my observations here 
along with L ibocedrus because these two genera seem to be very closely re- 
lated. Only one megaspore germinates, and this is accompanied by the usual 
rapid free nuclear division and the subsequent development of the large 
central vacuole, with a thin parietal layer of cytoplasm at the periphery. 
The formation of the parietal endosperm tissue takes place in identically 
the same manner as described above for Libocedrns . Fig. 15 represents 
a section of the lower half of a young prothallium with the primary 
prothallial cells very much elongated, and by their growth gradually 
