24 HERBERT SPENCER AND 



meres) are formed by the unequal division of the large 

 cells lA, iB, iC, iD. This new ' quartet ' of micromeres 

 we call 2a, 2b, 2c, and 2d, and it is noticeable that a great 

 part of the white material which formed the third polar 

 lobe passes into 2d (Fig. 9). It is known that the products 

 of this particular cell 2d form an important part of the 

 ventral surface of the posterior hemisphere of the larva 

 and of the trunk of the adult. Further than this we need 

 not go in the description of the normal course of develop- 

 ment. 



Now if the first polar lobe be removed, segmentation 

 continues, and ends in the production of a larva such 

 as is shown in Fig. 11. The upper hemisphere and the 

 three ciliated rings of the prototroch are there, but there 

 is no apical organ, and the lower hemisphere is aborted. 

 Such larvae are incapable of further development and 

 soon perish. 



It is clear that the material necessary for the formation 

 of the apical organ and the lower hemisphere (that is, 

 the material for the head and trunk of the adult), was 

 contained in the first polar lobe. 



If, in another experiment, the second polar lobe is 

 removed, the result is a larva such as is shown in Fig. 12. 

 The apical organ is present, but the lower hemisphere 

 is aborted. Clearly, when the first polar lobe was with- 

 drawn into CD, some part of its material, necessary for 

 the formation of the apical organ, must have passed out 

 of the lower hemisphere of that ceU, and have been 

 excluded from the second polar lobe — but the trunk- 

 forming material remains in the second polar lobe. 

 Further experiments prove that this is certainly the case, 

 and prove further that the material for the apical lobe is 

 first transferred to D, and then at the third division to id. 



For if the blastomeres are isolated, CD produces 

 a larva of normal aspect, but of reduced size ; AB produces 

 a larva similar to that of an ovum from which the first 

 polar lobe was removed. 



