482 PROCEEDINGS : BOSTON SOCIETY NATURAL HISTORY. 
When the conditions are favorable for the conversion of the myx- 
amoebae into spores, the pseudoplasmodium slowly ceases its upward 
movement. The exact cause of the cessation of this movement is 
as yet undetermined, but it is reasonable to believe that water has 
been lost by the component myxamoebae to such an extent that 
further active protoplasmic streaming is impossible. Frequently, 
vigorous looking colonies on the drier portions of a nutrient agar 
culture will cease movement and sporulate so quickly that the head 
does not assume its usual shape, but retains instead the elongated 
shape of the pseudoplasmodium. Certain species of Dictyostelium 
have a special tendency to develop thus prematurely when grown 
on a peptone culture medium. The spores of these abnormal sori 
may be also prematurely formed, for they then may be spherical 
instead of oval, and larger than the normal spore. 
The normal method of spore formation in the Dictyosteliaceae is 
as follows. Stalk formation ceases, the end of the axis frequently 
becomes broader and club shaped, and the colony moves upward a 
little higher and assumes a spherical shape. The end of the stalk 
does not then project above in the apical protuberance above men¬ 
tioned as it does in the ascending colony. The myxamoebae, dur¬ 
ing this stage, show dense unvacuolated contents, each being an oval 
or spherical body, separated from the surrounding individuals by a 
film which shows the characteristic mucous reaction with muchae- 
matin. A wall of cellulose is secreted about the protoplasm of 
each individual which thus becomes a refractive spore, very resistant 
to stains and other reagents. In Acrasis, according to van Tieghem,. 
this resistance is still further increased by the addition of a cuticu- 
larized external zone. The mucous substance between the spores 
cements them together, and the whole mass forms, with the excep¬ 
tion of Acrasis, a naked, deliquescent head. 
The size of the sorus is at first much increased by the moisture 
which it contains, but as the mass of spores dries, the sorus shrinks 
considerably. If the moisture be allowed to evaporate but slowly, 
the spores settle by gravity to the bottom of the globule, leaving 
a clear portion above. The heavy sori of luxuriant fructifications 
may even overcome the adhesive forces which hold them at the 
summit of their stalks and slide slowly down the support, when the 
naked apex then projects above the mass of spores. 
The size of the mature sori in all the members of the group varies 
