56 bulletin: museum of comparative zoology. 



the four-cell stage, i. e. tliey cross each other at approximately right 

 angles. A similar restoration to the conditions of the four-cell stage 

 occurs in Nereis, also in Umbrella at the twelve-cell stage, and probably 

 in Neritina. 



In Planorbis, however, according to Eabl's ('79) interpretation, the 

 cross furrow of the animal pole is not restored to the position of the four- 

 cell stage, but is turned 90° from it (see his Taf. XXXII. Figs. 10 A, 

 1 1 A). To accomplish this it is necessary for each of the cells of the 

 apical quartet (5.4) to be shifted 90° to the left, and thus completely 

 out of their own quadrants over upon the adjoining quadrants. It seems 

 very probable that Rabl is in error in this matter, and that in Planorbis, 

 as in the other forms, the division of this generation results in the 

 restoration of the cross furrows to the conditions of the four-cell stage. 



An examination of the sixteen-cell stage {cf. Figs. 21, 22) shows that 

 the cells of the apical quartet (5.4) lie in the same meridian as those of 

 the same quadrant in the basal quartet, i. e. a^'^ lies directly over cc'-^, 

 ¥■* over 6^^, etc. ; a'^, 6^-^, etc. lie to the left of the meridian of the quad- 

 rant to wliich they belong, and a^'^, L^\ etc. to the right. Thus of the 

 four granddaughter cells of the original blastomere occupying the quad- 

 rant, two only occupy the meridian corresponding to the middle of the 

 quadrant, the other two being placed laterally to it, one upon either side. 

 Similar conditions obtain in the corresponding stage of Nereis. In Neri- 

 tina, Planorbis, and Umbrella, the fact that the twelve-cell stage is 

 succeeded by the twenty-four-cell stage obscures somewhat the typical 

 arrangement, though it can still be traced. It will readily be seen that, 

 when the disturbing elements of unequal and non-synchronous division 

 are removed, this arrangement of the four granddaughter cells will hold 

 good for the descendants of ant/ blastomere in spiral cleavage, and that 

 normal and reversed cleavage will differ only in the transposition of the 

 lateral granddaughter cells; e. g. in the case under discussion a*^ and 

 a"' would be in a case of reversed cleavage transposed. Owing, perhaps, 

 to the unequal distribution of the yolk, this typical arrangement is not 

 found in the sixteen-cell stage of Chiton as figured by Kowalevsky ('83) 

 and Metcalf ('93), though it can be traced in the later stages. Metcalf 

 says of this phenomenon, " Uach cell then lies in the same meridian as 

 its grandparent, — a fact shown more clearly in tlie cleavage of such eggs 

 as those of Nereis and Crepidula." It is at once evident that this is 

 but a partial and misleading statement of the case, since it ignores the 

 fact that there are four fjranddayghter ceWs of every blastomere. It has 

 its explanation in the confusing custom adopted in previous systems of 



