CLEAVAGE AND DIFFERENTIATION 



III 



The third cleavage separates the first quartet of micromeres 

 (la, lb, ic, id) from the four macromeres {lA, iB, i C, iD). 



Fig. 53 

 Dentaliiim, development of isolated blastomeres. a. Larva resulting from an 

 isolated CD blastomere (therefore containing the first polar lobe) ; this larva is of 

 reduced size but normal in form except that the apical organ and post-trochal 

 region are proportionately too large, b, Twin larva to a, resulting from an isolated 

 AB blastomere from the same egg; this larva lacks the apical organ and post- 

 trochal region, c. Larva resulting from an isolated D blastomere (therefore con- 

 taining the second polar lobe) ; this larva is of reduced size but normal in form 

 except that the apical organ and post-trochal region are proportionately much too 

 large, d, Twin larva to c, resulting from an isolated C blastomere from the same 

 egg; this larva lacks the apical organ and post-trochal region, e, Larva resulting 

 from an isolated A or B blastomere ; this larva lacks the apical organ and post- 

 trochal region. /, Larva resulting from an isolated id blastomere; this larva 

 possesses the apical organ but lacks the post-trochal region, g, Twin larva to/, 

 resulting from an isolated i c blastomere from the same egg ; this larva lacks the 

 apical organ and post-trochal region. (From Jenkinson, Experimental Embryology, 

 Oxford, 1909, after Wilson.) 



Isolated la, ib, or ic blastomeres give larvae which possess a ring 

 of cilia, but lack gut, apical organ, and post-trochal region. An 

 isolated i d blastomere gives a similar larva, except that it possesses 



