9 6 



BIOLOGICAL LECTURES. 



annelids and molluscs, this "polar furrow" retains its original 

 direction until a late stage of the development, and is of great 

 service in orientation. 



At the next division each cell divides equatorially, so that 

 there results an eight-cell stage, having four cells above and 

 four below, the four upper being slightly smaller than the 

 lower. (See Fig. 3 of an eight-cell stage from below.) Note 

 especially the large size of D and the difference in size between 

 the upper and lower quartettes. Because of this difference in 

 size, we may speak of the upper quartette as micromeres, and 



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FIG. 3. A mphitrite, eight cells from below. FIG. 4. Podarke, eight cells from the side. 



the lower quartette as macromeres, the distinction between the 

 two quartettes being much less noticeable here than in some 

 other annelids. 



This process is repeated during the formation of two more 

 groups of micromeres, which form the body ectoderm, and a 

 fourth group, one member of which forms the mesoderm, while 

 the other three unite with the remaining macromeres to form 

 the entoderm. Meanwhile cells earlier formed have divided, 

 so that the separation of the mesoblast cell coincides with the 

 completiorr of the ideal sixty-four-cell stage, a stage which, 

 owing to delay in the division of some cells and a hastening 

 of the division of others, may never actually be attained. 



A marked feature of this formation of micromeres is the 

 division of each group in an alternating spiral direction. The 

 eight-cell stage (Fig. 3) arises from the four-cell by a division 



