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 coUenchymatous 

 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 Gonium. 



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 bi partitions 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, h, 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 h) 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 



