JENNIXGS : DEVELOPMENT OF ASPLANCHXA HEREICKII. 93 



of this first row of figures (73-80) of Melicerta is incorrectly given in 

 the " Erklarung der Abbilduugen." Thus, Figure 78 is said to be a 

 ** Dorsalansicht," which would bring the blastomere II (== C^) 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 Kerutheilung die Zelltheilung in dem 

 grossen Elastomer, die Theilungsebene steht senkrecht zur Kernspindel 

 und schneidet ein Stiick an der rechten Seite schief heraus (Fig. 78, 

 11)." ^ (p. 117.) Again, Figure 79 is said to be a ventral view, whereas 

 the same considerations as in the last case show it to be a doisal 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 Figm'e 79, though the latter itself is 

 said in the "Erklarung " 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 "'Erklarung der Abbildungen." 



In Melicerta, therefore, the polar cell is formed in the same position 

 as in Asplanchna Herrickii 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 npon the 

 surface of the egg. This is shown by Zelinka's figures as well as by his 

 descriptions. On page 62 he says, " Das Richtungskorperchen hat sei- 

 nen Platz, den es friiher eingenommen, verlassen und liegt nun ganz auf 

 den kleinen Zellen." The e^g of Callidina is enclosed by a rigid shell or 

 raembi-ane, 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 fornj and rotate within the shell. The original 

 place of polar-cell formation, with respect to this shell, is near one end, — 

 the end next to which lies later the anterior extremity of the embrj'o. 

 But immediately after the first cleavage the egg rotates within the shell 

 through an arc of about 90 degi-ees, and the region of polar-cell forma- 



^ The Italics are mine. 



