247 



The passage of the eight- into the sixteen-eelled stage, which in 

 the spiral type is always performed by a laeotropie cleavage, is 

 characterised in Volvoa^ by a progressive torsion of the cells of the 

 vegetative side with regard to those of the animal side and this in 



Fig. (>. Beginning of the 

 fourth cleavage, vegetative 

 side. 



Fig. 7. Transition 8 — 16, 

 animal side 



the same direction in which it has already manifested itself in the 

 foregoing cleavage, which is what we may call dexiotropic. This 

 expresses itself in the shape of the cells immediately when the fourth 

 cleavage sets in, as fig. 6 teaches us. The torsion here has already 

 become a little greater than 45° as becomes evident if we compare 

 the situation of the inferior parts of the vegetative cells {a,—(l,) 

 with regard to the cross of the cleavage furrows at the animal pole. 

 The fourth cleavage (fig. 7, 8), therefore, under the influence of 



Fig. 8. Transition 8—16, Fig. 9. Stage 16, 



vegetative side. animal side. 



the above torsion, must be described as dexiotropic. For this reason, 

 and in regard to the further cleavage, I think it inadvisable to 

 apply liere the nomenclature proposed by Conklin for the spiral 

 type, but will modify this a little. I call the cells of the four quadrants 

 resp. a, b, c, and d, and to their descendants J give each time the' 

 exponent 1 to the cell that lies to the animal side, and the exponent 

 2 to that lying to the vegetative side. Thus all the cells with the 

 letter a are descendants of the cell a of the four-celled stage, forming 



