56 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
the four-cell stage, i. e. they 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 Rabl’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, 
11A). 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 stago. 
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 a^, 
b4 over 1%, etc. ; a9?, 152, etc. lie to the left of the meridian of the quad- 
rant to which they belong, and a*”, 099, 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 any 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 arrangoment 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, “ Hach cell then lies in the same meridian as 
its grandparent, — a fact shown more clearly in the 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 granddaughter cells of every blastomere. It has 
its explanation in the confusing custom adopted in previous systems of 
