248 



together one quadrant which, moreover, I have surrounded with a 

 thick line in the figures. Now fig. 9 answers wholly to the image 

 presented by a 16-celled stage of the spiral cleavage type, however, 

 one would expect the cells a^\ n}"^ , 6'^ and 6" to represent together 

 one quadrant. This has been shown not to be the case, and if one takes 

 a view of figs. 8 and 9, the dexiotropic torsion that has occurred 

 during the cleavage, at once strikes the eye. Between a^^ and a" 

 this torsion now amounts to between 45° and 90°. The phialopore 

 is surrounded by the cells a" — (/*' and a^^ — c/^*, the former consti- 

 tuting the four longer, the latter the four shorter sides of the octangular 

 phialoporic border. Sometimes one of the cells (i"^ — d^^ also reaches 

 the border, the latter then being formed by nine cells. 



The fifth cleavage, leading to the 32-celled stage, is again laeotropic, 

 as is shown as well by a view from the animal (fig. 10) as from 

 the vegetative side (fig. 11). Thus the dexiotropic torsion is again 



(b"2) 



tc"^) 



(a'-) 



Fig. 10. Transition 16—32, 

 animal side. 



(d''^') 



Fig. 11. Transition 16—32, 

 vegetative side. 



continued here, and as fig. 11 and especially fig. 13 shows, this 

 torsion of the vegetative extremity of each quadrant with regard to 

 the animal extremity (the cell a^'') now amounts to nearly 90°. 

 While the cell a}^^ forces itself between a^"^ and «", as was already 

 the case in the 16-celled stage, so that a""^ is pushed aside a little 

 (figs. 11, 13), «■'' while dividing is pushed to the phialoporic border 

 by «^\ which also divides. As a consequence r/^" — c/^" now form 

 the longer, a"* — cZ"* the shorter sides of the octangular phialopore. 

 Often also all eight sides are of equal length. 



The arrangement of the cells in the 32-celled stage is so regular 

 (figs. 12 and 13) that no doubt one would not sus()ect from it the 

 torsion stated here by watching tlie cleavages. 



The last cleavage studied by me is the one that leads from the 

 32-celled to the 64-celled stage (figs. 14 and 15). The direction of 



