222 Nichols, A morphological study of Juniperus communis var. clepressv. 
that in Pinus the cells never reach the center “without having 
first divided by cell walls”, and tbat “a ring of tissue composed 
of longer or sborter cells is formed rather early in the inward 
growth of the prothallium”. This appears to be precisely what 
happens in the form under discussion, as may be seen from fig. 77. 
The further development of the prothallium was not followed in 
detail. Fig. 3, however, shows its appearance shortly after the 
cells have become closed in, and fig. 4 represents a similar section 
at about the time of fertilization. 
The phylogenetic importance of the megaspore membrane in 
Gymnosperms has been recently emphasized by Thomson (1905 a) 
who States that “the megaspore coat closely resembles that of a 
microspore both in its structure and its Chemical composition, and 
Fig. 4. 
similar to fig. 3 at ab out the 
time of fertilization. 
x 190. 
Fig. 3. 
Radial section through lower portion Section 
of prothallium shortly after formation 
thus affords additional evidence of the free-sporing nature of the 
ancestral forms of the Gymnosperms”. He finds that this structure is 
present in all the groups and sub-groups of these seed plants, except 
the Taxeae, from the ovule of whose forms it is entirelv or almost en- 
tirely eliminated. Thomson describes the megaspore membrane in J. 
sabina and J. virginiana, while Noren (1907) describes that of 
J. communis, and the observations of the writer are in accord 
with those of these two authors. The coat is formed during the 
free nuclear period of the embryo sac and reaches its highest 
development at about the time of fertilization. It is 2.5—3 u 
thick in the basal region of the prothallium and of fairly uniform 
thickness throughout, except at the micropylar end of the embryo 
sac where in the archegonial region it becomes quite thin. In 
section (fig. 73) two distinct layers are seen, the outermost of 
