156 BROOKLYN BOTANIC GARDEN MEMOIRS 
walls, as Meyer has most fully described, of the large masses of slime 
by which the protoplasts in the adult colony are surrounded. This 
slime, about whose nature Cohn, Klebs, Blochmann, and others have 
differed so widely, is, as is now generally recognized, the gelatinized 
cell wall comparable to the secondary thickenings in collenchymatous 
tissues through which extend the broad strands which provide for the 
again much disputed intercellular protoplasmic connections which 
are so conspicuous in the adult colonies. 
The firm adhesion of the daughter cells to each other and the re- 
establishment of the primitive cell division-growth rhythm are two 
further conditions to be reckoned with in the development of the 
colony of Volvox as compared with that of Goniwm. 
We may turn now to the morphogenetic processes involved in the 
reproduction of Volvox. Braun (’75) first clearly recognized the 
“division by the wheel-forming type”’ in Eudorina as distinguished 
from the ordinary successive bipartitions at right angles in Palmella, 
etc. Braun refers the readjustments of rounding up and rearrange- 
ment of the cells in forming the globular colony to the pressure of 
the developing slime envelopes. 
It has not been sufficiently emphasized that in the two-cell stage 
and in the four-cell stage, as in the cleavage of the animal egg, the 
halves and the quadrants respectively will tend by surface tension to 
round up and give us a plate-shaped instead of a globular mass. In 
Eudorina Goebel shows that the four cells tend to round up and are 
shortened. He further represents them as tending to remain at one 
end of the mother-cell cavity and to adhere to the surface of the 
mother-cell wall. This leads to a divergence of their major axes and 
(82, p. 36, Fig. 17) gives already at this stage a polar opening. Over- 
ton (’89, Taf. II, Fig. 10, a, 6, c) figures evidence of this divergence of 
the four cells and it has been observed by others. The third division 
by the wheel type, or radial type, gives us with the rounding up of 
the cells a disk consisting of four interior and four peripheral cells 
alternating with them, the familiar cross figure. The four inner cells 
may appear much larger and are commonly so figured. 
Overton (’89, Taf. II, Fig. 12, a and b) has shown very clearly 
that the apparent relative size of the central and peripheral cells 
varies with the level at which they are observed. The peripheral 
cells have slipped out of the plane of the central four so that the 
group of eight is already markedly concave. Two factors are pri- 
marily concerned in this displacement. First, the fact that we have 
by binary fission in two planes at right angles a group of eight rounded 
bodies which can form no stable least surface configuration in one 
plane and, second, the disk-shaped group formed by the rounding 
