90 BULLETIN : ML'SEUM OF COMPARATIVE ZOOLOGY, 



CuUidiua russeola and Discopus synaptse, according to Zelinka ('91, p. 53), 

 the polar cell is formed almost exactly at one of the ends of the ellipsoidal 

 egg, though a vei'y little to one side. This diflference 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 Asplauchna. 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. 



But 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 : — 



"Es verdient hervorgehoben zu werden, dass von dem Augenblicke 

 an, als das Eichtuugskorperchen gebildet wird, sammtliche Richtungen 

 im Raderthier-Eie orientirt sind. An dem Pol, in dessen Nahe das 

 Korperchen austritt, finden wir spater das Vorderende, am gegenliber- 

 liegenden das Hinterende, wahrend die Flache, in der es erscheiut, 

 zur Riickenflache wird." (Zelinka, '91, p. 54.) 



Accepting the later orientation of Zelinka, the above statement be- 

 comes accurate for Asplanchna Herrickii and Asplauchna priodonta if 

 •''Vorderende" and "Hinterende" are interchanged, and " Bauchflache " 

 is substituted for "Riickenflache"; 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, we 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 surfece of the animal, — the same form being re- 

 tained to a late stage, — yet during the processes of development that 



