200 THE FROG chap. 



The furrow which effects this division of the oosperm 

 passes through both black and white poles, so that each of 

 the two cells formed is half black and half white. Soon a 

 second furrow is formed at right angles to the first, being 

 like it, meridional, /.c, passing through the poles (Fig. 64, 

 B). It divides what we must now call the embryo into four 

 cells, each half black and half white. A third furrow is 

 then formed, passing round the equator, but nearer the 

 black than the white pole (C). It therefore divides the 

 embryo into eight cells, four upper black and four lower 

 white, the latter being obviously the larger. 



This process of division or segmentation of the oosperm 

 continues, the black cells dividing more rapidly than the white, 

 so that before long the embryo consists of a mass of cells, the 

 polyplast, somewhat resembling a mulberry, and therefore 

 often called a morula, one hemisphere being composed of 

 small cells containing much protoplasm and little yolk, and 

 externally pigmented (Fig. 64, D — F, mi), and the other 

 of larger cells containing little protoplasm and much yolk, 

 and not pigmented {^mg). As segmentation goes on, a 

 cavity appears in the interior of the embryo ; it is called 

 the segmentation cavity (E, /'/. del), and is due to the fact 

 that the work of segmentation produces a waste of sub- 

 stance, which there is at present no means of making good, 

 so that for a time, while the size of the embryo remains the 

 same, its bulk diminishes, some of its substance being used 

 up ; in other words, it feeds on itself 



Segmentation now proceeds to such an extent that the 

 black cells become too small to be seen except with a lens 

 of tolerably high magnifying power, so that with the amount 

 of magnification used in Fig. 64, G, the black hemisphere 

 shows no division into cells. At the same time the black 

 hemisphere gradually encroaches on the white until only a 



