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\ 

 lb ,1c , and Id^, 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-'), etc. The cells la^-ld^ divide into lower, 

 somewhat smaUer cells, la^^, etc., and upper larger cells la", etc.i 

 The cells la^-ld^ divide similarly, wten the egg in this stage is 



/sf quartette 



2nd 

 qua/Mis 



2nd 



quartette 



2nd 

 quartette 



Isf quartette 

 3A 



■^ quartette 



quarlette 3D 



Pig. 82. — Developing egg of Planocera inquilina. Thirtytwo-oell 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 



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

 an abbreviation for the whole quartette. Thus la^-ld?- means la', IJ', Ic', Id^ ; la'', 

 etc. likewise means the series la", 16", Ic", and Irf" 



