100 BULLETIN: MUSEUM OF COMPARATIVE ZOÜLOGY. 
observed by Tessin. IIe did not follow the divisions of the entoderm 
further. 
In Callidina a division takes place in the same manner as the seventh 
in Asplanchna (Figs. 64 and 65, Plate 8), separating % and de. The 
cleavage of de into det and dee 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 de. and 
die. 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 die, 
d??, d*?, and d now divides into three parts, but the details of these 
cleavages are not given. 
In Melicerta the cleavage of the entoderm is traced by Zelinka to a 
four-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 the 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 eell of the left posterior quadrant is 
envoloped by the other cells during the process of cleavage, and becomes 
the entoderm. 
C. Summary ON MATURATION AND CLEAVAGE IN THE ROTIFERA. 
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 blastoporie region, as stated by Zelinka for 
Callidina and Melicerta. 
9. A much greater regularity, and in a certain sense symmetry, are 
shown in the direction and rate of cleavage than has been shown for 
