539 
Dutt. — Pityostrobus macrocephalus , L. and H. 
way in which the ‘ tent-pole 5 of Ginkgo lifts up a cap formed by the remnant 
of the nucellus. The analogy, however, turns out to be quite false because 
from closer examination and comparison with other ovules it can be made 
out that the apical column or beak is composed of nucellar tissue, while the 
* cap ’ forms part of the separated inner layer of the integument. In the 
first place it is not difficult to recognize that the thick layer surrounding the 
prothallus consists of much more than the megaspore membrane alone. 
Presumably the latter does line the interior, since the whole structure is 
filled with remains of prothallial tissue, but it is fairly certain that the 
nucellar membrane is responsible for much of the thickness. This view 
derives much support from the fact that at both shoulders of the large 
prothallial body, part of the membrane is seen to become fused with the 
inner integumentary tissue. Moreover, where the structure of this 
membrane can be determined, as, for instance, in the apical region, it is 
found to share all the characteristics of the nucellar tissue of other ovules. 
We may conclude, therefore, that we have to deal with an ovule, at a fairly 
advanced stage of development, in which the prothallial tissue has become 
much enlarged and the nucellus reduced to a mere lining layer, only part of 
the original prolongation being still preserved. The nature of this prolonga- 
tion is discussed later on. Even in this ovule the nucellus has not altogether 
shrunk away from the integument. It is the inner layers of the latter which 
have separated from the main wall of the ovule, but yet adhere in several 
places to the nucellar tissue ; thus also taking some part in the formation of 
the composite lining of the prothallus. 
Turning now to other ovules for further verification, it may be noticed 
first that very similar conditions are to be found in one of the ovules 
directly below that just described. Here in one place, part of the rather 
delicate megaspore membrane has become separated from the thick sur- 
rounding layer, leaving no doubt that the latter is mainly of nucellar 
origin. In the other ovules shown in this longitudinal section of the 
cone, the prothallus, and consequently also the nucellus, has rarely such 
a well-rounded form as in two examples just cited ; both nucellus and 
prothallus are usually much shrunken. On the other hand, in another 
longitudinal section which is taken from the same cone, there are some 
well-preserved ovules in which the nucellus is in contact with the integu- 
ment for the greater part of its length (see Figs, n, 13, PI. XV). The 
nucellus lines the integument wall right up to the apical region, where 
it becomes free and extends across the cavity of the ovule. In both 
cases there is a large megaspore with a clearly defined membrane. In 
the ovule of Fig. 11, PI. XV, the inner layer of the integument has separated 
somewhat at the apex of the ovule. 
It is a significant fact that in the sections of P. ovatus those ovules 
which have been cut at all longitudinally exhibit very much the same 
