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, i.e., 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 proto- 

 plasm, and little yolk, and externally pigmented (D F, 

 mi), and the other of larger cells containing little proto- 

 plasm 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, bl. 

 ccel), and is due to the fact that the work of segmenta- 

 tion produces a waste of substance 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 



