ASPLANCHNA EBBESBORNII (ROTIFER) 405 



E- (fig. 62). Figure 63 represents a 250-cell stage, and by turn- 

 ing the embryo in different positions all of the cells can be recog- 

 nized, but it is impossible to tell the exact boundary of the cells 

 derived from any one quadrant. The derivatives of D always 

 take the initiative in division. Figure 63 represents the eighth 

 cleavage complete and the beginning of the ninth. In figure 64 

 the entodermal cells do not completely fill the central cavity of 

 the embryo. Next, the anterior entodermal cell E"^ divides very 

 unequally and forms E^-^ and e^- (fig. 65). The smaller cell, 

 after a few divisions, is difficult to follow. Immediately after the 

 division of the anterior cell E^ the cells E--^ and E^-^ divide 

 equally. The spindles of these divisions are shown in figure 65. 

 The division is equal and takes place in an anteroposterior 

 direction. The entoderm is now composed of five large cells and 

 three smaller ones. 



Figure 66 represents an embryo with the five large entodermal 

 cells viewed from the left side. Spindles are present in each of 

 the cells for the following cleavage. Figure 67 represents an 

 optical section a little earlier than the preceding, viewed from 

 the right side. The ectodermal cells are somewhat contracted 

 and do not fill the central cavity. The cells of the embryo at the 

 dorsal posterior end multiply very rapidly, extend backward 

 over the blastopore, and, in a later stage, contribute to the for- 

 mation of the temporary foot. At the next division each of the 

 five entodermal cells divides dorsoventrally and forms two 

 layers of five cells each, as shown in an optical section in figure 

 68, with the ectoderm removed. Figure 69 shows the same 

 stage with the ectoderm intact. At the next division each of 

 the five upper and the lower entodermal cells divide equally. 

 The cleavage occurs in an anteroposterior direction and produces 

 twenty large entodermal cells, as shown in figure 70, an upper 

 view. From this point forward no attempt was made to follow 

 the individual cleavage cells. Figure 71, a ten-hour embryo, 

 shows about the same stage as the preceding from the right side. 

 The position of the entoderm is indicated by a dotted outhne. 

 The beginning of the foot and the first stage in the formation of 

 the stomodaeum is evident from the ventral side. Figure 72, a 



JOURNAL OF MORPHOLOGY, VOL. 33, NO. 2 



