GROWTH AND DIVISION 79 



been invoked to explain cleavage in eggs. And the old notion of a 

 constriction band which must remain a complete ring in order to 

 exert a pull is of course precluded. There is no doubt that con- 

 striction occurs, which is probably from point to point on the 

 furrow. Localized contraction together with the cutting of the 

 lateral stripes and bands are probably the two agents directly 

 involved in fission. 



We now consider observations and experiments relating to the 

 question of how the division line is determined so that it should 

 be at a certain level on the cell, normally such as to produce 

 daughter cells of equal volume. This line appears to be precisely 

 laid out as a perfectly smooth curve without indefinite zig-zagging. 



Suggestively, there is a cell constituent which comes to follow 

 this contour during division. This is the complement of glycogenoid 

 granules which comprises the carbohydrate reserves of Stentor. 

 Weisz (1949a) had previously noted that these reserves are about 

 equally distributed between the two daughter cells though initially 

 lying at the posterior end, and later I supplied an exact account of 

 their distribution (Tartar, 1959a). In a well-fed pre-divisional 

 stentor the granules lie in a broad sub-cortical band at the posterior 

 end, exclusive of the pole itself and interrupted or missing in the 

 post-oral meridian (Fig. 17). About stage 5, considerably before 

 the first visible indication of a furrow, half of the granules migrate 

 forward. Those left behind become somewhat more diffuse than 

 they were before. The anterior border of the migrated complement 

 is also irregular, but its posterior boundary forms a sharp line 

 precisely defining the path of the fission fine which soon appears. 

 When furrowing occurs, therefore, it merely segregates the 

 reserves which were previously divided. These events do occur at 

 just the time when the fission fine is being determined with respect 

 to its location. But if immediately preceding this stage the carbo- 

 hydrate reserves are excised from the cell by cutting off the 

 posterior end where they still reside, division can still occur in their 

 absence. Therefore the peculiar behavior described is not the 

 cause but rather seems to be the sign of other factors which 

 locate the fission line. 



Whatever it is that determines the path of the division furrow, 

 there is further evidence of the pervasive nature of this agent from 

 unpublished experiments in which excision of parts far distant 



