100 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



observed by Tessiu. He did not follow the divisions of the entoderm 

 further. 



In Callidiua a division takes place in the same manner as the seventh 

 in Asplanchna (^Figs. G4 and 65, Plate 8), separating c?*-^ and d^-^. The 

 cleavage of d^-^ into d^-^ and d^-'- also follows, as described above for 

 Asplanchna. 



An unequal cleavage of d^-', as shown in Fig. 80 (Plate 10), was not 

 observed in Callidina. The cleavages which next ensue are described by 

 Zelinka as variable. The two cells corresponding to my d^-^ and d^'^ 

 divide in the same direction as the corresponding cleavages of Asplanchna 

 (Fig. 76, Plate 9, and Fig. 81, Plate 10), but the dorsal cells rf^^-^ and 

 rfio-* are smaller than the others. The cell d^-'^ (e, Zelinka) divides by 

 two successive divisions, at right angles to each other, into four cells; 

 one of these divisions corresponds to that indicated for this cell in Fig. 

 83 (Plate 10), while the other is at right angles to this. The order in 

 which these cleavages occur in Callidina is, however, variable. 



According to Zelinka, each of the four cells corresponding to my c?®-^, 

 d/'-'^, d^'^, and rf^** now divides into three parts, but the details of these 

 cleavages are not given. 



In ]\Ielicerta the cleavage of the entoderm is traced by Zelinka to a 

 foui*-cell condition, but the process is entirely different from that in 

 Asplanchna, Eosphora, and Callidina, so that it would not be of interest 

 to review tlie facts here. 



The process of gastrulation takes place in Callidina and Eosphora, and 

 probably in all other Rotifera, in a manner essentially similar to that in 

 Asplanchna ; the large ventral cell of the left posterior quadrant is 

 enveloped by the other cells during the process of cleavage, and becomes 

 the entoderm. 



C. Summary on Maturation and Cleavage in the Eotifera. 



In general, the following facts are shown for the early development 

 of Asplanchna, as compared with previous accounts of the development 

 of Rotifera. 



1. The polar cell is formed at the animal pole of the egg, at a point 

 opposite that where the blastopore is later found, and not at the dorsal 

 or anterior margin of the blastoporic region, as stated by Zelinka for 

 Callidina and Melicerta. 



2. A much greater regularity, and in a certain sense symmetry, are 

 shown in the direction and rate of cleavage than has been shown for 



