22 HERBERT SPENCER AND 



relatively less and less important and eventually dis- 

 appear. 



In the egg of this animal a broad middle band of red 

 pigment separates a colourless upper polar area from 

 a colourless lower polar area. The lower polar area 

 consists of a dense granular and apparently homogeneous 

 protoplasm in which there are no yolk corpuscles — 

 in this it differs from the rest of the egg. 



When the egg is fertilized and about to divide it 

 protrudes from its lower pole a lobe, into which nearly 

 all the white matter of the lower polar area passes (Fig. 2). 

 Division then takes place in such a way that the ' polar 

 lobe ' is attached to only one of the two cells formed by 

 the division (Fig. 3). When division is accomplished 

 this ' first polar lobe ' is drawn up again into the 

 blastomere to which it is attached. The white polar 

 area is confined to the larger of the two cells. We 

 call the smaller cell AB, the- larger cell with the white 

 lower polar area CD. Before the next division takes 

 place the lower polar area is again extruded in the form of 

 a 'second polar lobe ' (Fig. 4). Each blastomere is divided 

 into two, and the second polar lobe remains attached 

 to one of the cell products into which CD has divided. 

 We call this product D, and the embryo now consists 

 •of four cells, A, B, C, D. The second polar lobe is once 

 more withdrawn into D, but before the third division 

 takes place the white matter is once more extruded as the 

 ' third polar lobe ' (Fig. 7). In the third division each of the 

 blastomeres, A, B, C, D, divides unequally, giving rise 

 to four smaller cells above which we call la, ib, ic, id, 

 and four larger cells below which we call lA, iB, iC, iD. 

 The third polar lobe remains attached to iD, and when 

 division is complete is withdrawn into it. Prior to the 

 fourth division the white material of the third polar 

 lobe moves through the blastomere iD and is mostly 

 aggregated near its upper surface. The fourth division 

 resembles the third, inasmuch as four small cells (micro- 



