Cytology of a new variety of Achlya americana . 161 
sents uninucleate oospores from similar preparations of 
eight-day cultures. 
The examination of many hundreds of oogonia and oospores 
in preparations from six, seven, and eight-day cultures leave 
little doubt in my mind that the approximation of the nuclei 
in the five-day culture preparations is temporary, and that the 
fusion does not take place until about three days later, when 
the nuclei have been for some time in the resting condition. 
It would be interesting to determine the conditions which 
regulate this long-delayed fusion of the gameto-nuclei. It 
appears to be of common occurrence amongst the Thallophytes. 
During the three days which elapse before fusion takes 
place, the oospore undergoes important changes : the proto- 
plasm contracts somewhat, and the cell-wall thickens ; the 
fat-globules get larger, leaving behind them bigger cavities 
in the protoplasm, and little groups of microsomata make 
their appearance, recalling the much larger structures seen in 
the oospores of Saprolegnia . Fortunately, however, in Achlya , 
there is no structure present which prevents the accurate 
examination of the nuclei, and for this reason alone it is 
satisfactory that the study of the plant was carried to an end. 
In cultures nine days old (Fig. 51) a new structure makes 
its appearance in the form of an additional thick inner wall 
showing a faint stratification. This wall is obviously built 
up of reserve material, as it disappears early in germination. 
Between it and the outer wall (in balsam mounts) there is 
always an empty space : this is no doubt due to the greater 
contraction which it undergoes during dehydration. If the 
sections are examined in water or in iodine solution, the outer 
and inner walls are in close contact, but are readily recog- 
nizable by the difference in colour and refractive power. 
Testing the sections for cellulose with iodine and sulphuric 
acid, the inner thick wall becomes a deep blue at once, and 
swells up quickly, frequently pressing out the protoplasm 
and completely filling the cavity ; the inner layers of the 
outer wall sVell considerably and become of a greenish blue 
colour, while the outermost layer retains the yellow colour 
M 
