90 BULLETIN: MUSEUM OF COMPARATIVE ZOÜLOGY. 
Callidina russeola and Discopus synaptie, according to Zelinka (91, p. 53), 
the polar cell is formed almost exactly at one of the ends of the ellipsoidal 
egg, though a very little to one side. This difference is a point of little 
or no significance; an examination of Zelinka’s figures (Taf. I. Figg. 1-5) 
shows that the polar cell in Callidina russeola occupies the same position 
with regard to the axis of the first cleavage spindle as it does in the three 
species of Asplanchna. Possibly the egg of Callidina russeola is forced 
by the shell to take such a form that the axes of the egg, as indicated by 
the first cleavage spindle, do not coincide with the apparent axes indi- 
cated by the shape. The place of formation of the polar cell, as might 
be expected, is correlated with the axis indicated by the cleavage spindle. 
After the first cleavage a rotation occurs in Callidina russeola, bringing 
the apparent axis into agreement with the real axis. 
Dut with regard to the place of polar-cell formation in its relation to 
the orientation of the egg as shown by later development, a remarkable 
disagreement exists between the condition in Asplanchna Herrickii and 
Asplanchna priodonta on the one hand, and the description given by 
Zelinka of Callidina russeola on the other. The following is Zelinka’s 
statement of the orientation of the egg of Callidina with relation to the 
place of polar-cell formation : — 
* Ws verdient hervorgehoben zu werden, dass von dem Augenblicke 
an, als das Richtungskörperchen gebildet wird, sämmtliche Richtungen 
im Räderthier-Eie orientirt sind. An dem Pol, in dessen Nähe das 
Körperchen austritt, finden wir später das Vorderende, am gegenüber- 
liegenden das Hinterende, während die Fläche, in der es erscheint, 
zur Rückenfläche wird." (Zelinka, ’91, p. 54.) 
Accepting the later orientation of Zelinka, the above statement be- 
comes accurate for Asplanchna Herrickii and Asplanchna priodonta if 
** Yorderende " and *Hinterende " are interchanged, and ** Bauchfläche ” 
is substituted for “Rückenfläche”; in other words, the orientation 
of Asplanchna with reference to the polar cell is precisely the opposite 
of that of Callidina. The statement for Asplanchna would read: “ At 
the pole in the neighborhood of which the polar cell appears, wo find 
later the posterior end, at the opposite pole the anterior end, while the 
surface on which it appears becomes the ventral surface.” This state- 
ment, while correct if we relate the orientation of the animal simply to 
the form of the egg, as is done by Zelinka, contains one false implication. 
While that surface of the egg on which the polar cell is formed does later 
become the ventral surface of the animal,— the same form being re- 
tained to a late stage, — yet during the processes of development that 
