PLATYHELMINTHES 



107 



are nearly as large as the macromeres, but in the European 

 genus Discocoelis the inequality in size between micromeres and 

 macromeres is much more marked. In the next period of cell- 

 division the second micromeres are formed by a laeotropic division. 



The first quartette divides so as to give rise to two tiers of four 

 cells, all being of nearly equal size. The lower are of course la 2 , 

 1& 2 , lc 2 , and l5 2 , and only divide once or twice again. 



The third quartette is then formed, and the second quartette 

 divides, each member giving rise to an upper smaller and a lower 

 larger cell (2a 2 ), etc. The cells la 1 -!^ 1 divide into lower, 

 somewhat smaller cells, la 12 , etc., and upper larger cells la 11 , etc. 1 

 The cells la 2 - Id 2 divide similarly. When the egg in this stage is 



2nd 



quartette 



2nd ^ 

 quarfeife 



2nd 

 quartelti 



^_ 1st quartette 

 X/5A 



2nd 

 qwzrle&e 



I N^lf^Hpl 



2nd ^^' 

 quartette 3D 



FIG. 82. Developing egg of Planocera inquilina. Thirtytwo-cell stage viewed from 

 vegetative pole. (After Surface.) 



viewed from its lower or vegetative pole, the cells belonging 

 to the second and third quartettes are seen to alternate with one 

 another and together to form a girdle round the egg just below its 

 equator (Fig. 82). 



After the 32 -cell stage, distinct periods of division, affecting 

 the cells of the egg simultaneously, cease, and hence the different 

 groups of cells must be dealt with separately. 



An extraordinary feature in Planocera is the formation of the fourth 

 quartette. In this case Surface still terms the upper daughter a 

 micromere and the lower a macromere, in order to facilitate com- 

 parison with other eggs of the same type of cleavage. But in 

 the formation of this quartette the daughter cells, the so-called 

 "macromeres" (mac, Fig. 84), occupying the vegetative pole of 



1 The hyphen between the names of two cells indicates that these names are 

 an abbreviation for the whole quartette. Thus la 1 -!^ 1 means la 1 , 16 1 , lc 1 , Id 1 ; la 11 , 

 etc. likewise means the series la 11 , 16 11 , lc 11 , and Id 11 



