245 



to occur in considerable number in the Victoria regia-b^sin of the 

 Leyden botanical garden, [ readily seized it. The study was made 

 on living material. During development the parthenogonidium, which 

 continues to communicate by plasmodesms with the surrounding 

 cells, considerably increases in size '), so that the older stages are 

 often easier to study than younger ones, for which the use of oil- 

 immersion as a rule is to be preferred. 



By two meridional cleavages the parthenogonidium is first divided 

 into four equal cells, which each will give rise to a quadrant. The 

 eight-celled stage has already been figured repeatedly for Volvo.r 

 and other Volvocinea, but not the transition of the four- into the 



Fig. 2. Beginning of 

 the third cleavage, animal 

 side 



Fig. 3. Transition 4 — 8, 

 vegetative side 



eight-celled stage. Figs. 1, 2, and 3 teach us that during this cleavage 

 a torsion amounting to 45° occurs between what we may call for 

 the sake of shortness the four vegetative cells and the four animal 

 cells. In the terminology of the spiral cleavage type we should call 

 this torsion a dexiotropic one since, if we look at the egg from the 

 side of the animal pole, the four animal cells appear to lie to the 

 right of the four lower cells. 



It seemed to me interesting to make out if this third cleavage 

 always takes place in the same way or if, as could equally be 

 imagined, it is sometimes dexiotropic and sometimes laeotropic. In 

 the cleavage of Balanus, which shows a similar torsion, I found e.jj. 

 both possibilities occurring indisci'iminately '). In the spiralcleavagr' 

 type the third cleavage is always dexiotropic with the exception of 

 inversely wound Gasteropoda where the whole cleavage proceeds iji 

 an inverse manner. So not onl}' the adult form but equally the 

 earliest cleavage stages present the reflected image of what we find 

 in dextral Gasteropods. 



ij All the figures in tliis article have been drawn the same size. 

 2) H. C. Delsman, 11)17. Die Embryonalenlwicklung von Balanus balanoides Linn 

 Tijdschr. Nederl. Dierk. Ver. (2), Dl. 15. 



