ASPLANCHNA EBBESBORNII (rOTIFER) 397 



at an angle of about 45° with that of the first. The two cells 

 divide at different times. These two cleavages combined corre- 

 spond to the second cleavage as it occurs in many of the annelids 

 and molluscs. The cell CD divides first into two very unequal 

 parts (figs. 18 and 19). The division of AB is nearly equal 

 (figs. 18 to 20). Shortly after the second cleavage, a slight 

 shifting occurs as the cells flatten (figs. 19 to 24). The largest 

 cell, D, is posterior, B median anterior, C right, and B left (fig. 

 21), with reference to the median axis of the future animal. 

 The largest cell, D, always divides first in the formation of the 

 quartettes. The orientation of the four-celled embryo is very 

 simple and agrees with that of the annelids at the same stage of 

 development. From this point forward, however, the position 

 taken by the resultant cleavage cells is no longer comparable 

 with that of the annelids, but the sequence of cleavages in the 

 following stages is very similar. 



Third cleavage: In the formation of the first generation of 

 ectomeres (d^, b^ &, aO, the cell D divides first, the new cell is 

 budded off in a dorsal anterior direction on the median dorsal 

 side of A, B, and C (text fig. b and figs. 23 and 24), making a 

 five-cell stage. In this process the polar body is carried forward 

 to the anterior end with the cell d^ While the cleavage spindle is 

 forming for the production of the micromere d^, the cells A, B, and 

 C elongate in an anteroposterior direction (text fig. c and figs. 

 24 and 25), so that the cleavage plane of A, B, or C is not hori- 

 zontal, but in a dorsoventral direction at right angles to the 

 long axis of the embryo (figs. 26 to 31). Thus, instead of having 

 the micromeres above the macromeres as in many forms, the 

 cells a^, h\ c\ and d^ are on the same level with A, B, and C (figs. 

 31 and 32). The division of the macromeres A, B, and C is 

 nearly equal in the formation of the first generation of ectomeres. 

 They do not divide simultaneously, but in the invariable order 

 C, B, A. Thus there occurs successively a six-, seven-, and 

 eight-cell stage (figs. 30 to 32). In the formation of the eight- 

 cell stage B and b^ are pressed ventrally by d^ (figs. 24 and 31). 

 In the eight-cell stage d^ is median dorsal, B and b^ median 

 ventral, C and c^ right, A and a^ left, and D posterior extending 



