250 



We liave seen, however, that the cleavage of Volvox may not be 

 counted as belonging to the spiral type in the form in whichlt occurs in 

 Metazoa, though the arrangement of the cells is more in a spiral 

 than it is witli the latter. Though there are certain points in common 

 1 yet refrain from further speculations in this direction. 



In another respect, however, the results reached seem to me to 

 be interesting. We have been able to state during the cleavage a 

 progressive torsion of the vegetative cells with regard to the animal 

 cells which becomes especially manifest from a coniparison of ligs. 

 2, 4, 7, 9, 10, 12, and 14 and of figs. 3, 5, 6, 8, 11, 13, and 15. 

 So we have every reason to assume that in the adult form also a similar 

 arrangement of the cells prevails. In the spiral cleavage typo the 

 succeeding dexiotropic and laeotropic divisions nearly annul the effect 

 of each other, so th'it in the blastula the cells belonging to one 

 quadrant nearly occupy an area situated between two meridians 

 distant 90° from each other, as is represented fig. 16a. Fig. 166 

 shows I he situation of the cells belonging to one quadrant in the 

 case where the torsion does not surpass 90°. How great the latter 

 has become in the adult Tiq/vö.?; cannot be made out. Now in the same 

 dexiotropic rlirection also the rotation occurs, as we have seen, and 

 it seems to me probable that in this case we may look for a more 

 direct relation between the two phenomena than with the torsion of 

 Gasteropods. Let us assume to this end that not oidy the colony but 

 also each of the cells of Volvo.c has a certain polarity and thus a 

 main axis in the direction from the animal to the vegetative pole of 

 the colony. This polarity of the cells e.g. manifests itself in the 

 corresponding direction in which all the flagella beat causing a water 

 current from in front backwards, which makes the organism move 



Fig. 16/^. 



in the direction of the animal pole. If a Volvo.v be pressed between 

 a cover-slip and an object-slide so that it cannot move anymore, this 



