398 GEORGE W. TANNREUTHER 



dorsoventral (figs. 31 to 33). The micromeres form the anterior 

 end. In a few instances, C, B, and A divided in a nearly hori- 

 zontal plane, thus placing the micromeres above the parent cells 

 instead of on the same level with them (text figs, d and e and 

 fig. 33). This mode of division, however, is very unusual, but 

 is comparable with that of the annehds and polyclades. 



At the completion of the eight-cell stage, the embryo often 

 assumes the shape of the one-cell condition and the cleavage 

 furrows are scarcely distinguishable. The embryo at the dif- 

 ferent stages of development is very plastic and may asssume 

 almost any shape under abnormal pressure. If the egg be 

 removed from the reproductive organs, with the egg membrane 

 intact, after cleavage has begun, normal development will 

 continue. By slight pressure the individual cells of the eight-cell 

 stage can be separated. The isolated cells seldom continue to 

 divide, but begin to deteriorate almost immediately. 



Fourth cleavage (sixteen-cell stage) : A nine-cell stage is reached 

 by the formation of d^ from the large cell D on the median dorsal 

 side, in an anterior direction (figs. 34 and 35). As d^ is formed, 

 d^ is carried around the dorso-anterior end. Next, d^ divides in 

 an anteroposterior direction (fig. 36). Following this, the 

 macromeres A, B, and C and their micromeres divide in an 

 anteroposterior direction. The division occurs in the order C 

 and c^ B and b^ A and a^ thus producing a twelve-, fourteen-, 

 and sixteen-cell stage, respectively (figs. 34 to 38). The embryo 

 is now composed of four rows of cells with four cells in each row 

 (figs. 38 to 44). 



The fifth cleavage: The derivatives of D divide in the follow- 

 ing order: d^, d^, d^-\ thus producing a seventeen-, an eighteen-, 

 and a nineteen-cell stage (figs. 45 to 48). The cleavage spindles 

 in the C, B, and A rows indicate the direction of the cleavages 

 in passing from the nineteen- to the thirty-one-cell stage (figs. 

 45 to 48) . In a few of the embryos, as in annelids, D budded off 

 a third cell, d^ (figs. 46, 47, 49, and 50). This extra cell, when 

 produced, has no special significance in the future development 

 of the individuals which bear it; d=' and its derivatives will not 

 be considered in the further description of the cleavage stages. 



