IV. i INITIAL STRUCTURE OF THE GERM 225 



these missing parts are never regenerated ; the pretrochal region 

 is, however, increased and the three rows of cilia in the proto- 

 troch are usually developed. 



The polar lobe is thus clearly connected with the formation of 

 the trunk and the preoral sense-organ. 



The segmentation of the isolated blastomeres is partial, as in 

 the types already examined, but their subsequent behaviour 

 depends on the presence or absence of the polar lobe. AB larvae 

 or A, B, or C larvae closely resemble the deficient trochophore 

 just described ; CD or D larvae are normal, though the structures 

 derived from the polar lobe are out of proportion to the rest 



FIG. lZ5.Dentalium. a, Normal trochophore of 24 hours ; b, larva of 

 24 hours after removal of the first polar lobe. (After Wilson, 1904.) 



(Fig. 136 a-d, g}. The failure of the egg to develop normally is 

 not due to insufficiency of material, for the volumes of CD and 

 D are less than that of a whole but lobeless egg. 



The micromeres 1 , 1 b, \c, when isolated, become actively 

 swimming, ectoblastic embryos provided with a prototroch, but 

 with no apical organ, gut, or post-trochal region; the other 

 micromere, 1 d, however, has an apical organ, though it is still 

 without the hinder region and incapable of gastrulation (Fig. 

 l3Qe,f). It appears, therefore, that the material for the apical 

 organ is now placed in this cell, and this is proved by experiment ; 

 for if the polar lobe is cut away during the second cleavage, or 



JENKINSON 



