9 6 



UNEQUAL SEGMENTATION. 



pole, the latter the lower. The ovum is composed of protoplasm 

 containing in suspension numerous yolk-spherules. The largest 



FIG. 40. SEGMENTATION OF COMMON FROG. RANA TEMPORARIA. (Copied 

 from Ecker.) 



The numbers above the figures refer to the number of segments at the stage figured. 



of these are situated at the lower pole, the smaller ones at the 

 upper pole, and the smallest of all in the peripheral layer of the 

 upper pole, in which also pigment is scattered and causes the 

 black colour visible from the surface. 



The first formed furrow is a vertical furrow. It commences 

 in the upper half of the ovum, through which it extends rapidly, 

 and then more slowly through the lower. As soon as the first 

 furrow has extended through the egg, and the two halves have 

 become separated from each other, a second vertical furrow 

 appears at right angles to the 

 first and behaves in the same 

 way (fig. 40, 4). 



The next furrow is equa- 

 torial or horizontal (fig. 40, 8). 

 It does not arise at the true 

 equator of the egg, but much 

 nearer to its upper pole. It 

 extends rapidly round the egg 

 and divides each of the fourpre- 

 vious segments into two parts, 

 one larger and one smaller. 

 Thus at the end of this stage 

 there are present four small 

 and four large segments. At 

 the meeting point of these a 



II 



FIG. 41. SECTION THROUGH FROG'S 



OVUM AT THE CLOSK OK SKGMKNTATION. 



sg. segmentation cavity. //. large yolk-con- 

 taining cells, ep. small cells at formative 

 pole (epiblast). 



