JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRIOKII. 93 
of this first row of figures (73-80) of Melicerta is incorrectly given in 
tho * Erklärung der Abbildungen." Thus, Figure 78 is said to be a 
* Dorsalansicht,” which would bring the blastomere II (= Q9?) upon 
the left side, But this blastomere is, in all other rotifers whose develop- 
ment is known, formed on the right side, and Zelinka states concerning 
this very figure, “ Zuerst folgt der Kerntheilung die Zelltheilung in dem 
grossen Blastomer, die Theilungsebene steht, senkrecht zur Kernspindel 
und schneidet ein Stück an der rechten Seite schief heraus (Fig. 78, 
J). 1 (p. 117.) Again, Figure 79 is said to be a ventral view, whereas 
tho same considerations as in tho last case show it to be a dorsal view. 
Figure 80 is said to be a ventral view of the stage shown in Figure 78. 
Yet it is evidently a later stage than Figure 78 ; it really represents a 
ventral view of a stage similar to Figure 79, though the latter itself is 
said in the “Erklärung” to be a ventral view. In the text, Zelinka 
states correctly that Figure 79 is a dorsal, Figure 80 a ventral view 
(p. 118) of the same stage (p. 117), contrary to the statements in 
the “ Erklärung der Abbildungen.” 
In Melicerta, therefore, the polar cell is formed in the same position 
as in Asplanchna Herriekii and Asplanchna priodonta, and marks the 
animal pole of the egg. 
It seems very improbable that between these three forms and Calli- 
dina there should actually be such a difference in regard to the place of 
polar-cell formation as is brought out by the above comparisons. Ze- 
linka’s account of Callidina is full and clear upon this point ; in both 
his description and his figures the polar cell is traced to a late stage, 
when a mistake in the orientation is impossible. There seems, however, 
to be one opportunity for error. The polar cell in Callidina is not em- 
bedded between the blastomeres, as in Asplanchna, but lies free upon the 
surface of the egg. This is shown by Zelinka’s figures as well as by his 
descriptions. On page 62 he says, “ Das Richtungskörperchen hat sei- 
non Platz, den es früher eingenommen, verlassen und liegt nun ganz auf 
den kleinen Zellen.” The egg or Callidina is enclosed by a rigid shell or 
membrane, which is separated from the egg itself by a space, except at 
the sides. ‘The shell retains constantly the ellipsoidal form, while the 
egg itself may change its form and rotate within the shell. The original 
place of polar-coll formation, with respect to this shell, is near one end, — 
the end next to which Les later the anterior extremity of the embryo. 
But immediately after the first cleavage the egg rotates within the sbell 
through an are of about 90 degrees, and the region of polar-cell forma- 
1 The Italics are mine. 
