68 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
of the left posterior one, i. e. if we adopt Wilson's orientation instead of 
Rabl’s. In the table given above it will be noticed that the mesoderm 
cell M is placed as a member of the basal quartet of the sixth generation. 
It should be remembered, however, that Rabl in a later paper ('80) 
revised this earlier opinion as to the origin of the mesoderm cell, deriving 
CLEAVAGE OF PLANORBIS. 
Rabl (79). 
RABL’S NOMENCLATURE, REVISED NOMENCLATURE, 
Geneve Number 
cies ceu o ation, Cells. | EAN 3. 
Cells. Cells, Spirals. 
A ni E E a^ 
V 24 47 ^g € 5.2 a 
RE ; 24 | a9 os Left? 
1T ^19 19 20 
VI. F 
Ey Ey Lys Bis 24 diro «ii Left. 
EJ, EJ, EJ, M 
U 1, 1, 5,4 
E, E, E, E, 24 at c Right. 
Ey Ey Lio En E 
y 
4 3 5.2 
E, E, E, E, 12 ANS y Right. 
EJ, EJ, EJ, ME 
E, E, E, E, qu 
1 2 3 4 
EL EJ, EJ, MB IV. $ A< qua | Left. 
EJ, EJ, EJ, ME III. 4 | A, BO, D. Right. 
II. 2 AB, OD. 
I. 1 Ovum. 
it this time by division from the cell M of the above table. According 
to this the mesoderm cell in Planorbis, as in many other forms, belongs 
to the quartet 7.2. It seems to me that the orientation of the stages of 
Planorbis previous to the formation of the mesoderm is still an open 
question, As I have stated in the discussion of Limax, the orientation 
