HARPER: BINARY FISSION AND SURFACE TENSION 163 



Biitschli regards it as an open question whether all the cells divide in 

 the later stages. It is, however, obvious that successive bipartition 

 of all the cells is the natural method of maintaining the globular form 

 already achieved. Any excess or deficiency of the number of divi- 

 sions in any considerable group of cells would manifest itself at once 

 as a bulge or depression in the surface of the expanding sphere unless 

 compensatory divisions elsewhere and far-reaching, gliding move- 

 ments of the cells among one another were possible. There is no 

 evidence either of the occurrence or the possibility of such movements. 

 As has been many times observed, the daughters of the original group 

 of four can be recognized late in the life of the colony in their original 

 positions with respect to each other and the colony as a whole. 



Oltmanns ('04) follows Goebel ('82) and Goroschankin ('75) in 

 asserting that this original polar group of four cells does not divide 

 after the sixteen-cell stage but does not give any very positive evidence 

 on the point. Overton ('89, Taf. Ill, Fig. 18) in a colony of about 

 200 cells shows a group of eight cells, six of which are dividing. It is, 

 of course, obvious that a failure of the original group of four to con- 

 tinue dividing after the sixteen-celled stage would not prevent the 

 maintenance of the rounded form of the colony in case there were 

 compensating divisions in the adjacent cells. 



It is interesting to note that Kirchner ('79) finds the development 

 of the colony from the fertilized egg of V. aureus essentially like that 

 of the asexual germ cell. His figures give some indication of the 

 elongation of the cells before division and he describes the cup-shaped 

 form of the colony in the eight-cell stage. 



It seems to me that we are justified in concluding that Volvox, 

 though showing deep-seated specialization of somatic and germ cells 

 in which it contrasts markedly with Eudorina, Pandorina and Gonium, 

 still like them shows vegetative totipotency and equivalence of its 

 cells in the growth of the colony. This is an important consideration 

 in view of the question as to the origin of the differentiation of germ 

 and somatic cells which is so conspicuous in the adult colony of Vol- 

 vox, and entirely lacking in the simpler members of the series. This 

 differentiation is such that the germ cells are distributed solely in a 

 single half or three fourths of the colony, the remaining portions re- 

 maining persistently somatic-sterile. The fertile area of the colony is 

 regularly the posterior half or three fourths as the colony swims. 

 It would be of great theoretical interest in this earliest appearance of 

 the differentiation of soma and germ plasm if it could be shown that 

 the cells bearing the germ plasm were different in cell lineage, age 

 since last division, relative maturity as indicated by total number of 

 divisions undergone, or in any other way, from the remaining cells of 



