38 
Blackman and Fraser . — Further Studies on the 
area (PI. Ill, Figs. 1-4). It is only in exceptional cases or in material in 
which the cytoplasm has been much overstained that a nuclear reticulum 
can be clearly made out (Fig. 8). 
As development of the young aeddium proceeds the cells with 
dense contents become somewhat larger, and their origin from hyphal 
rows of cells less obvious. The next stage is that among the mass 
of dense uninucleate cells a few binucleate ones appear, and these have 
increased somewhat in size (Fig. 2). More and more cells gradually 
become binucleate and arrange themselves into a definite layer. These 
binucleate cells are the ‘ fertile cells ’ of the aecidium, and it is from them 
that the rows of aecidiospores and intercalary cells are derived, the fertile 
cells forming the basal layer of the aecidium (Fig. 7). 
Before they become binucleate the fertile cells are not very clearly 
defined, but at the stage of development, at which some of the cells with 
dense contents are changing from the uninucleate condition to that with 
paired nuclei, there can be observed migrations of nuclei from one cell into 
a cell which is still uninucleate. These nuclear migrations are much more 
difficult to observe than in the case of Phragmidinm violaceum , for the 
‘ fertile cells ’ are neither well defined when young, nor can progressive 
stages of development be observed in passing from the periphery to the 
centre of the aecidium. Thus migrations have to be sought for generally 
in the mass of mixed uninucleate and binucleate cells such as is found in 
the stage shown in Fig. 2. 
Only a small number of migrations were clearly observed, but there 
can be little doubt that, as in Phragmidium violaceum (where numerous 
cases were observed), the condition of conjugate nuclei in the aecidium is 
normally brought about in this form by a process of nuclear migration into 
the special fertile cells. 
The migrations were of the same type as those observed earlier in 
Phragmidium , the nucleus becoming very much constricted in its passage 
through the wall. In the present form, however, the nucleolus, being the 
only deeply staining portion of the nucleus, is the part that can be traced 
most easily through the wall. During the process it becomes stretched out 
into a band-like structure (Figs. 3-6) (often with a characteristic beaked 
appearance) in the formation of which the chromatin, no doubt, plays 
a small part. 
No signs were observed of the fusion of the fertile cells in pairs as 
described by Christman, although on the appearance of that author’s paper 
the preparations were carefully re-examined with that object in view. 
Owing to the difficulty of distinguishing the fertile cells in the very 
young state the presence of definite sterile cells, as in Phragmidium , could 
not be made out with any certainty, although indications of them were 
observed in a few cases. 
